diff --git a/app/assets/javascripts/discourse/app/components/discourse-topic.js b/app/assets/javascripts/discourse/app/components/discourse-topic.js
index 3e9906351b3..8ef20067199 100644
--- a/app/assets/javascripts/discourse/app/components/discourse-topic.js
+++ b/app/assets/javascripts/discourse/app/components/discourse-topic.js
@@ -2,6 +2,7 @@ import { getOwner } from "@ember/application";
 import Component from "@ember/component";
 import { alias } from "@ember/object/computed";
 import { schedule, scheduleOnce, throttle } from "@ember/runloop";
+import { inject as service } from "@ember/service";
 import { isBlank } from "@ember/utils";
 import $ from "jquery";
 import ClickTrack from "discourse/lib/click-track";
@@ -23,6 +24,7 @@ export default Component.extend(Scrolling, MobileScrollDirection, {
     "topic.category.read_restricted:read_restricted",
     "topic.deleted:deleted-topic",
   ],
+  header: service(),
   menuVisible: true,
   SHORT_POST: 1200,
 
@@ -54,6 +56,7 @@ export default Component.extend(Scrolling, MobileScrollDirection, {
 
   _hideTopicInHeader() {
     this.appEvents.trigger("header:hide-topic");
+    this.header.topic = null;
     this._lastShowTopic = false;
   },
 
@@ -62,6 +65,7 @@ export default Component.extend(Scrolling, MobileScrollDirection, {
       return;
     }
     this.appEvents.trigger("header:show-topic", topic);
+    this.header.topic = topic;
     this._lastShowTopic = true;
   },
 
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-header.gjs b/app/assets/javascripts/discourse/app/components/glimmer-header.gjs
new file mode 100644
index 00000000000..9573233f960
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-header.gjs
@@ -0,0 +1,225 @@
+import Component from "@glimmer/component";
+import { tracked } from "@glimmer/tracking";
+import { action } from "@ember/object";
+import didInsert from "@ember/render-modifiers/modifiers/did-insert";
+import { inject as service } from "@ember/service";
+import { modifier } from "ember-modifier";
+import concatClass from "discourse/helpers/concat-class";
+import scrollLock from "discourse/lib/scroll-lock";
+import DiscourseURL from "discourse/lib/url";
+import { scrollTop } from "discourse/mixins/scroll-top";
+import and from "truth-helpers/helpers/and";
+import not from "truth-helpers/helpers/not";
+import or from "truth-helpers/helpers/or";
+import AuthButtons from "./glimmer-header/auth-buttons";
+import Contents from "./glimmer-header/contents";
+import HamburgerDropdownWrapper from "./glimmer-header/hamburger-dropdown-wrapper";
+import Icons from "./glimmer-header/icons";
+import SearchMenuWrapper from "./glimmer-header/search-menu-wrapper";
+import UserMenuWrapper from "./glimmer-header/user-menu-wrapper";
+
+const SEARCH_BUTTON_ID = "search-button";
+
+let _customHeaderClasses = [];
+export function addCustomHeaderClass(className) {
+  _customHeaderClasses.push(className);
+}
+
+export default class GlimmerHeader extends Component {
+  @service router;
+  @service search;
+  @service currentUser;
+  @service site;
+  @service appEvents;
+  @service register;
+  @service header;
+
+  @tracked skipSearchContext = this.site.mobileView;
+  @tracked panelElement;
+
+  appEventsListeners = modifier(() => {
+    this.appEvents.on(
+      "header:keyboard-trigger",
+      this,
+      this.headerKeyboardTrigger
+    );
+    return () => {
+      this.appEvents.off(
+        "header:keyboard-trigger",
+        this,
+        this.headerKeyboardTrigger
+      );
+    };
+  });
+
+  get customHeaderClasses() {
+    return _customHeaderClasses.join(" ");
+  }
+
+  @action
+  headerKeyboardTrigger(msg) {
+    switch (msg.type) {
+      case "search":
+        this.toggleSearchMenu();
+        break;
+      case "user":
+        this.toggleUserMenu();
+        break;
+      case "hamburger":
+        this.toggleHamburger();
+        break;
+      case "page-search":
+        if (!this.togglePageSearch()) {
+          msg.event.preventDefault();
+          msg.event.stopPropagation();
+        }
+        break;
+    }
+  }
+
+  @action
+  toggleSearchMenu() {
+    if (this.site.mobileView) {
+      const context = this.search.searchContext;
+      let params = "";
+      if (context) {
+        params = `?context=${context.type}&context_id=${context.id}&skip_context=${this.skipSearchContext}`;
+      }
+
+      if (this.router.currentRouteName === "full-page-search") {
+        scrollTop();
+        document.querySelector(".full-page-search").focus();
+        return false;
+      } else {
+        return DiscourseURL.routeTo("/search" + params);
+      }
+    }
+
+    this.search.visible = !this.search.visible;
+    if (!this.search.visible) {
+      this.search.highlightTerm = "";
+      this.search.inTopicContext = false;
+      document.getElementById(SEARCH_BUTTON_ID)?.focus();
+    }
+  }
+
+  @action
+  togglePageSearch() {
+    this.search.inTopicContext = false;
+
+    let showSearch = this.router.currentRouteName.startsWith("topic.");
+    // If we're viewing a topic, only intercept search if there are cloaked posts
+    if (showSearch) {
+      const controller = this.register.lookup("controller:topic");
+      const total = controller.get("model.postStream.stream.length") || 0;
+      const chunkSize = controller.get("model.chunk_size") || 0;
+      showSearch =
+        total > chunkSize &&
+        document.querySelector(
+          ".topic-post .cooked, .small-action:not(.time-gap)"
+        )?.length < total;
+    }
+
+    if (this.search.visible) {
+      this.toggleSearchMenu();
+      return showSearch;
+    }
+
+    if (showSearch) {
+      this.search.inTopicContext = true;
+      this.toggleSearchMenu();
+      return false;
+    }
+
+    return true;
+  }
+
+  @action
+  toggleUserMenu() {
+    this.header.userVisible = !this.header.userVisible;
+    this.toggleBodyScrolling(this.header.userVisible);
+    this.args.animateMenu();
+  }
+
+  @action
+  toggleHamburger() {
+    if (this.args.sidebarEnabled && !this.site.narrowDesktopView) {
+      this.args.toggleSidebar();
+      this.args.animateMenu();
+    } else {
+      this.header.hamburgerVisible = !this.header.hamburgerVisible;
+      this.toggleBodyScrolling(this.header.hamburgerVisible);
+      this.args.animateMenu();
+    }
+  }
+
+  @action
+  toggleBodyScrolling(bool) {
+    if (!this.site.mobileView) {
+      return;
+    }
+    scrollLock(bool);
+  }
+
+  @action
+  setPanelElement(element) {
+    this.panelElement = element;
+  }
+
+  <template>
+    <header
+      class={{concatClass this.customHeaderClasses "d-header"}}
+      {{this.appEventsListeners}}
+    >
+      <div class="wrap">
+        <Contents
+          @sidebarEnabled={{@sidebarEnabled}}
+          @toggleHamburger={{this.toggleHamburger}}
+          @showSidebar={{@showSidebar}}
+        >
+          {{#unless this.currentUser}}
+            <AuthButtons
+              @showCreateAccount={{@showCreateAccount}}
+              @showLogin={{@showLogin}}
+              @canSignUp={{@canSignUp}}
+            />
+          {{/unless}}
+          {{#if
+            (not (and this.siteSettings.login_required (not this.currentUser)))
+          }}
+            <Icons
+              @sidebarEnabled={{@sidebarEnabled}}
+              @toggleSearchMenu={{this.toggleSearchMenu}}
+              @toggleHamburger={{this.toggleHamburger}}
+              @toggleUserMenu={{this.toggleUserMenu}}
+              @searchButtonId={{SEARCH_BUTTON_ID}}
+              @panelElement={{this.panelElement}}
+            />
+          {{/if}}
+
+          {{#if this.search.visible}}
+            <SearchMenuWrapper @closeSearchMenu={{this.toggleSearchMenu}} />
+          {{else if this.header.hamburgerVisible}}
+            <HamburgerDropdownWrapper
+              @toggleHamburger={{this.toggleHamburger}}
+            />
+          {{else if this.header.userVisible}}
+            <UserMenuWrapper @toggleUserMenu={{this.toggleUserMenu}} />
+          {{/if}}
+
+          <div id="additional-panel-wrapper" {{didInsert this.setPanelElement}}>
+          </div>
+
+          {{#if
+            (and
+              (or this.site.mobileView this.site.narrowDesktopView)
+              (or this.header.hamburgerVisible this.header.userVisible)
+            )
+          }}
+            <div class="header-cloak"></div>
+          {{/if}}
+        </Contents>
+      </div>
+    </header>
+  </template>
+}
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-header/auth-buttons.gjs b/app/assets/javascripts/discourse/app/components/glimmer-header/auth-buttons.gjs
new file mode 100644
index 00000000000..2de9f1130e6
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-header/auth-buttons.gjs
@@ -0,0 +1,28 @@
+import Component from "@glimmer/component";
+import { inject as service } from "@ember/service";
+import DButton from "discourse/components/d-button";
+import and from "truth-helpers/helpers/and";
+import not from "truth-helpers/helpers/not";
+
+export default class AuthButtons extends Component {
+  @service header;
+
+  <template>
+    <span class="header-buttons">
+      {{#if (and @canSignUp (not this.header.topic))}}
+        <DButton
+          class="btn-primary btn-small sign-up-button"
+          @action={{@showCreateAccount}}
+          @label="sign_up"
+        />
+      {{/if}}
+
+      <DButton
+        class="btn-primary btn-small login-button"
+        @action={{@showLogin}}
+        @label="log_in"
+        @icon="user"
+      />
+    </span>
+  </template>
+}
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-header/contents.gjs b/app/assets/javascripts/discourse/app/components/glimmer-header/contents.gjs
new file mode 100644
index 00000000000..429af099f2d
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-header/contents.gjs
@@ -0,0 +1,61 @@
+import Component from "@glimmer/component";
+import { hash } from "@ember/helper";
+import { inject as service } from "@ember/service";
+import and from "truth-helpers/helpers/and";
+import BootstrapModeNotice from "../bootstrap-mode-notice";
+import MountWidget from "../mount-widget";
+import PluginOutlet from "../plugin-outlet";
+import SidebarToggle from "./sidebar-toggle";
+import TopicInfo from "./topic/info";
+
+export default class Contents extends Component {
+  @service site;
+  @service currentUser;
+  @service siteSettings;
+  @service header;
+
+  get topicPresent() {
+    return !!this.header.topic;
+  }
+
+  <template>
+    <div class="contents">
+      {{#if this.site.desktopView}}
+        {{#if @sidebarEnabled}}
+          <SidebarToggle @toggleHamburger={{@toggleHamburger}} />
+        {{/if}}
+      {{/if}}
+
+      <div class="home-logo-wrapper-outlet">
+        <PluginOutlet @name="home-logo-wrapper">
+          <MountWidget
+            @widget="home-logo"
+            @args={{hash minimized=this.topicPresent}}
+          />
+        </PluginOutlet>
+      </div>
+
+      {{#if this.header.topic}}
+        <TopicInfo @topic={{this.header.topic}} />
+      {{else if
+        (and
+          this.siteSettings.bootstrap_mode_enabled
+          this.currentUser.staff
+          this.site.desktopView
+        )
+      }}
+        <div class="d-header-mode">
+          <BootstrapModeNotice />
+        </div>
+      {{/if}}
+
+      <div class="before-header-panel-outlet">
+        <PluginOutlet
+          @name="before-header-panel"
+          @outletArgs={{hash topic=this.header.topic}}
+        />
+      </div>
+      <div class="panel" role="navigation">{{yield}}</div>
+    </div>
+  </template>
+}
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-header/dropdown.gjs b/app/assets/javascripts/discourse/app/components/glimmer-header/dropdown.gjs
new file mode 100644
index 00000000000..895099bd6bb
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-header/dropdown.gjs
@@ -0,0 +1,57 @@
+import Component from "@glimmer/component";
+import { hash } from "@ember/helper";
+import { on } from "@ember/modifier";
+import { action } from "@ember/object";
+import concatClass from "discourse/helpers/concat-class";
+import { wantsNewWindow } from "discourse/lib/intercept-click";
+import icon from "discourse-common/helpers/d-icon";
+import i18n from "discourse-common/helpers/i18n";
+import and from "truth-helpers/helpers/and";
+import closeOnClickOutside from "../../modifiers/close-on-click-outside";
+
+export default class Dropdown extends Component {
+  @action
+  click(e) {
+    if (wantsNewWindow(e)) {
+      return;
+    }
+    e.preventDefault();
+    this.args.onClick(e);
+
+    // remove the focus of the header dropdown button after clicking
+    e.target.tagName.toLowerCase() === "button"
+      ? e.target.blur()
+      : e.target.closest("button").blur();
+  }
+
+  <template>
+    <li
+      class={{concatClass
+        @className
+        (if @active "active")
+        "header-dropdown-toggle"
+      }}
+      {{(if
+        (and @active @targetSelector)
+        (modifier
+          closeOnClickOutside @onClick (hash targetSelector=@targetSelector)
+        )
+      )}}
+    >
+      <button
+        class="button icon btn-flat"
+        aria-expanded={{@active}}
+        aria-haspopup="true"
+        href={{@href}}
+        data-auto-route="true"
+        title={{i18n @title}}
+        aria-label={{i18n @title}}
+        id={{@iconId}}
+        {{on "click" this.click}}
+      >
+        {{icon @icon}}
+        {{@contents}}
+      </button>
+    </li>
+  </template>
+}
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-header/hamburger-dropdown-wrapper.gjs b/app/assets/javascripts/discourse/app/components/glimmer-header/hamburger-dropdown-wrapper.gjs
new file mode 100644
index 00000000000..63ce833708c
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-header/hamburger-dropdown-wrapper.gjs
@@ -0,0 +1,76 @@
+import Component from "@glimmer/component";
+import { hash } from "@ember/helper";
+import { on } from "@ember/modifier";
+import { action } from "@ember/object";
+import { prefersReducedMotion } from "discourse/lib/utilities";
+import { isTesting } from "discourse-common/config/environment";
+import discourseLater from "discourse-common/lib/later";
+import closeOnClickOutside from "../../modifiers/close-on-click-outside";
+import HamburgerDropdown from "../sidebar/hamburger-dropdown";
+
+export default class HamburgerDropdownWrapper extends Component {
+  @action
+  click(e) {
+    e.preventDefault();
+    if (
+      e.target.closest(
+        ".sidebar-section-header-button, .sidebar-section-link-button, .sidebar-section-link"
+      )
+    ) {
+      this.args.toggleHamburger();
+    }
+  }
+
+  @action
+  clickOutside(e) {
+    if (
+      e.target.classList.contains("header-cloak") &&
+      !prefersReducedMotion()
+    ) {
+      const panel = document.querySelector(".menu-panel");
+      const headerCloak = document.querySelector(".header-cloak");
+      const finishPosition =
+        document.querySelector("html").classList["direction"] === "rtl"
+          ? "340px"
+          : "-340px";
+      panel
+        .animate([{ transform: `translate3d(${finishPosition}, 0, 0)` }], {
+          duration: 200,
+          fill: "forwards",
+          easing: "ease-in",
+        })
+        .finished.then(() => {
+          if (isTesting()) {
+            this.args.toggleHamburger();
+          } else {
+            discourseLater(() => this.args.toggleHamburger());
+          }
+        });
+      headerCloak.animate([{ opacity: 0 }], {
+        duration: 200,
+        fill: "forwards",
+        easing: "ease-in",
+      });
+    } else {
+      this.args.toggleHamburger();
+    }
+  }
+  <template>
+    <div
+      class="hamburger-dropdown-wrapper"
+      {{! template-lint-disable no-invalid-interactive }}
+      {{on "click" this.click}}
+      {{! we don't want to close the hamburger dropdown when clicking on the hamburger dropdown itself
+        so we use the secondaryTargetSelector to prevent that }}
+      {{closeOnClickOutside
+        this.clickOutside
+        (hash
+          targetSelector=".hamburger-panel"
+          secondaryTargetSelector=".hamburger-dropdown"
+        )
+      }}
+    >
+      <HamburgerDropdown />
+    </div>
+  </template>
+}
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-header/icons.gjs b/app/assets/javascripts/discourse/app/components/glimmer-header/icons.gjs
new file mode 100644
index 00000000000..7a9c01563a6
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-header/icons.gjs
@@ -0,0 +1,73 @@
+import Component from "@glimmer/component";
+import { inject as service } from "@ember/service";
+import getURL from "discourse-common/lib/get-url";
+import not from "truth-helpers/helpers/not";
+import or from "truth-helpers/helpers/or";
+import MountWidget from "../mount-widget";
+import Dropdown from "./dropdown";
+import PanelPortal from "./panel-portal";
+import UserDropdown from "./user-dropdown";
+
+let _extraHeaderIcons = [];
+export function addToHeaderIcons(icon) {
+  _extraHeaderIcons.push(icon);
+}
+
+export function clearExtraHeaderIcons() {
+  _extraHeaderIcons = [];
+}
+
+export default class Icons extends Component {
+  @service site;
+  @service currentUser;
+  @service header;
+  @service search;
+
+  _isStringType = (icon) => typeof icon === "string";
+
+  <template>
+    <ul class="icons d-header-icons">
+      {{#each _extraHeaderIcons as |Icon|}}
+        {{#if (this._isStringType Icon)}}
+          <MountWidget @widget={{Icon}} />
+        {{else}}
+          {{#let
+            (component PanelPortal panelElement=@panelElement)
+            as |panelPortal|
+          }}
+            <Icon @panelPortal={{panelPortal}} />
+          {{/let}}
+        {{/if}}
+      {{/each}}
+
+      <Dropdown
+        @title="search.title"
+        @icon="search"
+        @iconId={{@searchButtonId}}
+        @onClick={{@toggleSearchMenu}}
+        @active={{this.search.visible}}
+        @href={{getURL "/search"}}
+        @className="search-dropdown"
+        @targetSelector=".search-menu-panel"
+      />
+
+      {{#if (or (not @sidebarEnabled) this.site.mobileView)}}
+        <Dropdown
+          @title="hamburger_menu"
+          @icon="bars"
+          @iconId="toggle-hamburger-menu"
+          @active={{this.header.hamburgerVisible}}
+          @onClick={{@toggleHamburger}}
+          @className="hamburger-dropdown"
+        />
+      {{/if}}
+
+      {{#if this.currentUser}}
+        <UserDropdown
+          @active={{this.header.userVisible}}
+          @toggleUserMenu={{@toggleUserMenu}}
+        />
+      {{/if}}
+    </ul>
+  </template>
+}
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-header/panel-portal.gjs b/app/assets/javascripts/discourse/app/components/glimmer-header/panel-portal.gjs
new file mode 100644
index 00000000000..59dd0c4620a
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-header/panel-portal.gjs
@@ -0,0 +1,9 @@
+import ConditionalInElement from "../conditional-in-element";
+
+const PanelPortal = <template>
+  <ConditionalInElement @element={{@panelElement}}>
+    {{yield}}
+  </ConditionalInElement>
+</template>;
+
+export default PanelPortal;
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-header/search-menu-wrapper.gjs b/app/assets/javascripts/discourse/app/components/glimmer-header/search-menu-wrapper.gjs
new file mode 100644
index 00000000000..e6b3559c5ee
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-header/search-menu-wrapper.gjs
@@ -0,0 +1,9 @@
+import SearchMenuPanel from "../search-menu-panel";
+
+const SearchMenuWrapper = <template>
+  <div class="search-menu glimmer-search-menu" aria-live="polite">
+    <SearchMenuPanel @closeSearchMenu={{@closeSearchMenu}} />
+  </div>
+</template>;
+
+export default SearchMenuWrapper;
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-header/sidebar-toggle.gjs b/app/assets/javascripts/discourse/app/components/glimmer-header/sidebar-toggle.gjs
new file mode 100644
index 00000000000..2049316b979
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-header/sidebar-toggle.gjs
@@ -0,0 +1,37 @@
+import Component from "@glimmer/component";
+import { on } from "@ember/modifier";
+import { action } from "@ember/object";
+import { inject as service } from "@ember/service";
+import concatClass from "discourse/helpers/concat-class";
+import icon from "discourse-common/helpers/d-icon";
+import i18n from "discourse-common/helpers/i18n";
+
+export default class SidebarToggle extends Component {
+  @service site;
+
+  @action
+  toggleWithBlur(e) {
+    this.args.toggleHamburger();
+    // remove the focus of the header dropdown button after clicking
+    e.target.tagName.toLowerCase() === "button"
+      ? e.target.blur()
+      : e.target.closest("button").blur();
+  }
+
+  <template>
+    <span class="header-sidebar-toggle">
+      <button
+        title={{i18n "sidebar.title"}}
+        class={{concatClass
+          "btn btn-flat btn-sidebar-toggle no-text btn-icon"
+          (if this.site.narrowDesktopView "narrow-desktop")
+        }}
+        aria-expanded={{if @showSidebar "true" "false"}}
+        aria-controls="d-sidebar"
+        {{on "click" this.toggleWithBlur}}
+      >
+        {{icon "bars"}}
+      </button>
+    </span>
+  </template>
+}
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-header/topic/featured-link.gjs b/app/assets/javascripts/discourse/app/components/glimmer-header/topic/featured-link.gjs
new file mode 100644
index 00000000000..9ca13c9c83b
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-header/topic/featured-link.gjs
@@ -0,0 +1,26 @@
+import Component from "@glimmer/component";
+import { inject as service } from "@ember/service";
+import { extractLinkMeta } from "discourse/lib/render-topic-featured-link";
+import icon from "discourse-common/helpers/d-icon";
+
+export default class FeaturedLink extends Component {
+  @service header;
+
+  get meta() {
+    return extractLinkMeta(this.header.topic);
+  }
+
+  <template>
+    {{#if this.meta}}
+      <a
+        class="topic-featured-link"
+        rel={{this.meta.rel}}
+        target={{this.meta.target}}
+        href={{this.meta.href}}
+      >
+        {{icon "external-link-alt"}}
+        {{this.meta.domain}}
+      </a>
+    {{/if}}
+  </template>
+}
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-header/topic/info.gjs b/app/assets/javascripts/discourse/app/components/glimmer-header/topic/info.gjs
new file mode 100644
index 00000000000..81ec91dc8da
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-header/topic/info.gjs
@@ -0,0 +1,182 @@
+import Component from "@glimmer/component";
+import { fn, hash } from "@ember/helper";
+import { on } from "@ember/modifier";
+import { action } from "@ember/object";
+import { inject as service } from "@ember/service";
+import { htmlSafe } from "@ember/template";
+import categoryLink from "discourse/helpers/category-link";
+import concatClass from "discourse/helpers/concat-class";
+import renderTags from "discourse/lib/render-tags";
+import DiscourseURL from "discourse/lib/url";
+import icon from "discourse-common/helpers/d-icon";
+import i18n from "discourse-common/helpers/i18n";
+import and from "truth-helpers/helpers/and";
+import gt from "truth-helpers/helpers/gt";
+import not from "truth-helpers/helpers/not";
+import or from "truth-helpers/helpers/or";
+import PluginOutlet from "../../plugin-outlet";
+import FeaturedLink from "./featured-link";
+import Participant from "./participant";
+import Status from "./status";
+
+export default class Info extends Component {
+  @service currentUser;
+  @service site;
+  @service siteSettings;
+
+  get showPM() {
+    return !this.args.topic.is_warning && this.args.topic.isPrivateMessage;
+  }
+
+  get totalParticipants() {
+    return (
+      (this.args.topic.details.allowed_users?.length || 0) +
+      (this.args.topic.allowed_groups?.length || 0)
+    );
+  }
+
+  get maxExtraItems() {
+    return this.args.topic.tags?.length > 0 ? 5 : 10;
+  }
+
+  get twoRows() {
+    return (
+      this.tags?.length ||
+      this.showPM ||
+      this.siteSettings.topic_featured_link_enabled
+    );
+  }
+
+  get tags() {
+    if (this.args.topic.tags) {
+      return renderTags(this.args.topic);
+    }
+  }
+
+  get remainingParticipantCount() {
+    return this.totalParticipants - this.maxExtraItems;
+  }
+
+  get participants() {
+    const participants = [
+      ...this.args.topic.details.allowed_users,
+      ...this.args.topic.details.allowed_groups,
+    ];
+    return participants.slice(0, this.maxExtraItems);
+  }
+
+  @action
+  jumpToTopPost(e) {
+    e.preventDefault();
+    if (this.args.topic) {
+      DiscourseURL.routeTo(this.args.topic.firstPostUrl, {
+        keepFilter: true,
+      });
+    }
+  }
+
+  <template>
+    <div
+      class={{concatClass (if this.twoRows "two-rows") "extra-info-wrapper"}}
+    >
+      <div class={{concatClass (if this.twoRows "two-rows") "extra-info"}}>
+        <div class="title-wrapper">
+          <h1 class="header-title">
+            {{#if this.showPM}}
+              <a
+                class="private-message-glyph-wrapper"
+                href={{fn this.currentUser.pmPath @topic}}
+                aria-label={{i18n "user.messages.inbox"}}
+              >
+                {{icon "envelope" class="private-message-glyph"}}
+              </a>
+            {{/if}}
+
+            {{#if (and @topic.fancyTitle @topic.url)}}
+              <Status @topic={{@topic}} @disableActions={{@disableActions}} />
+
+              <a
+                class="topic-link"
+                {{on "click" this.jumpToTopPost}}
+                href={{@topic.url}}
+                data-topic-id={{@topic.id}}
+              >
+                <span>{{htmlSafe @topic.fancyTitle}}</span>
+              </a>
+
+              <span class="header-topic-title-suffix">
+                <PluginOutlet
+                  @name="header-topic-title-suffix"
+                  @outletArgs={{hash topic=@topic}}
+                />
+              </span>
+            {{/if}}
+          </h1>
+
+          {{#if (or @topic.details.loaded @topic.category)}}
+            {{#if
+              (and
+                @topic.category
+                (or
+                  (not @topic.category.isUncategorizedCategory)
+                  (not this.siteSettings.suppress_uncategorized_badge)
+                )
+              )
+            }}
+              <div class="categories-wrapper">
+                {{#if @topic.category.parentCategory}}
+                  {{#if
+                    (and
+                      @topic.category.parentCategory.parentCategory
+                      (not this.site.mobileView)
+                    )
+                  }}
+                    {{categoryLink
+                      @topic.category.parentCategory.parentCategory
+                    }}
+                  {{/if}}
+
+                  {{categoryLink
+                    @topic.category.parentCategory
+                    (hash hideParent="true")
+                  }}
+                {{/if}}
+                {{categoryLink @topic.category}}
+              </div>
+            {{/if}}
+
+            <div class="topic-header-extra">
+              {{htmlSafe this.tags}}
+              <div class="topic-header-participants">
+                {{#if this.showPM}}
+                  {{#each this.participants as |participant|}}
+                    <Participant
+                      @user={{participant}}
+                      @type={{if participant.username "user" "group"}}
+                      {{! username for user, name for group }}
+                      @username={{or participant.username participant.name}}
+                    />
+                  {{/each}}
+
+                  {{#if (gt this.totalParticipants this.maxExtraItems)}}
+                    <a
+                      class="more-participants"
+                      {{on "click" this.jumpToTopPost}}
+                      href={{@topic.url}}
+                      data-topic-id={{@topic.id}}
+                    >
+                      +{{this.remainingParticipantCount}}
+                    </a>
+                  {{/if}}
+                {{/if}}
+              </div>
+              {{#if this.siteSettings.topic_featured_link_enabled}}
+                <FeaturedLink />
+              {{/if}}
+            </div>
+          {{/if}}
+        </div>
+      </div>
+    </div>
+  </template>
+}
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-header/topic/participant.gjs b/app/assets/javascripts/discourse/app/components/glimmer-header/topic/participant.gjs
new file mode 100644
index 00000000000..efec25e8197
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-header/topic/participant.gjs
@@ -0,0 +1,50 @@
+import Component from "@glimmer/component";
+import { concat, hash } from "@ember/helper";
+import { on } from "@ember/modifier";
+import { action } from "@ember/object";
+import { inject as service } from "@ember/service";
+import avatar from "discourse/helpers/bound-avatar-template";
+import icon from "discourse-common/helpers/d-icon";
+import getURL from "discourse-common/lib/get-url";
+import eq from "truth-helpers/helpers/eq";
+
+export default class Participant extends Component {
+  @service appEvents;
+
+  get url() {
+    return this.args.type === "user"
+      ? this.args.user.path
+      : getURL(`/g/${this.args.username}`);
+  }
+
+  @action
+  click(e) {
+    this.appEvents.trigger(
+      `topic-header:trigger-${this.args.type}-card`,
+      this.args.username,
+      e.target
+    );
+    e.preventDefault();
+  }
+
+  <template>
+    <span class={{concat "trigger-" @type "-card"}}>
+      <a
+        class="icon"
+        {{on "click" this.click}}
+        href={{this.url}}
+        data-auto-route="true"
+        title={{@username}}
+      >
+        {{#if (eq @type "user")}}
+          {{avatar @user.avatar_template "tiny" (hash title=@username)}}
+        {{else}}
+          <span>
+            {{icon "users"}}
+            {{@username}}
+          </span>
+        {{/if}}
+      </a>
+    </span>
+  </template>
+}
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-header/topic/status.gjs b/app/assets/javascripts/discourse/app/components/glimmer-header/topic/status.gjs
new file mode 100644
index 00000000000..415982bf76f
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-header/topic/status.gjs
@@ -0,0 +1,62 @@
+import Component from "@glimmer/component";
+import { on } from "@ember/modifier";
+import { action } from "@ember/object";
+import { inject as service } from "@ember/service";
+import concatClass from "discourse/helpers/concat-class";
+import TopicStatusIcons from "discourse/helpers/topic-status-icons";
+import { escapeExpression } from "discourse/lib/utilities";
+import icon from "discourse-common/helpers/d-icon";
+import I18n from "discourse-i18n";
+
+export default class Status extends Component {
+  @service currentUser;
+
+  get canAct() {
+    return this.currentUser && !this.args.disableActions;
+  }
+
+  get topicStatuses() {
+    let topicStatuses = [];
+    TopicStatusIcons.render(this.args.topic, (name, key) => {
+      const iconArgs = { class: key === "unpinned" ? "unpinned" : null };
+      const statusIcon = { name, iconArgs };
+
+      const attributes = {
+        title: escapeExpression(I18n.t(`topic_statuses.${key}.help`)),
+      };
+      let klass = ["topic-status"];
+      if (key === "unpinned" || key === "pinned") {
+        klass.push("pin-toggle-button", key);
+        klass = klass.join(" ");
+      }
+      topicStatuses.push({ attributes, klass, icon: statusIcon });
+    });
+
+    return topicStatuses;
+  }
+
+  @action
+  togglePinnedForUser(e) {
+    if (!this.canAct) {
+      return;
+    }
+    const parent = e.target.closest(".topic-statuses");
+    if (parent?.querySelector(".pin-toggle-button")?.contains(e.target)) {
+      this.args.topic.togglePinnedForUser();
+    }
+  }
+
+  <template>
+    <span class="topic-statuses">
+      {{#each this.topicStatuses as |status|}}
+        {{! template-lint-disable no-invalid-interactive }}
+        <span
+          class={{concatClass status.klass "topic-status"}}
+          {{on "click" this.togglePinnedForUser}}
+        >
+          {{icon status.icon.name class=status.icon.iconArgs.class}}
+        </span>
+      {{/each}}
+    </span>
+  </template>
+}
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-header/user-dropdown.gjs b/app/assets/javascripts/discourse/app/components/glimmer-header/user-dropdown.gjs
new file mode 100644
index 00000000000..c87d17575ed
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-header/user-dropdown.gjs
@@ -0,0 +1,53 @@
+import Component from "@glimmer/component";
+import { concat } from "@ember/helper";
+import { on } from "@ember/modifier";
+import { action } from "@ember/object";
+import { inject as service } from "@ember/service";
+import concatClass from "discourse/helpers/concat-class";
+import { wantsNewWindow } from "discourse/lib/intercept-click";
+import i18n from "discourse-common/helpers/i18n";
+import or from "truth-helpers/helpers/or";
+import Notifications from "./user-dropdown/notifications";
+
+export default class UserDropdown extends Component {
+  @service currentUser;
+
+  @action
+  click(e) {
+    if (wantsNewWindow(e)) {
+      return;
+    }
+    e.preventDefault();
+    this.args.toggleUserMenu();
+
+    // remove the focus of the header dropdown button after clicking
+    e.target.tagName.toLowerCase() === "button"
+      ? e.target.blur()
+      : e.target.closest("button").blur();
+  }
+
+  <template>
+    <li
+      id="current-user"
+      class={{concatClass
+        (if @active "active")
+        "header-dropdown-toggle current-user user-menu-panel"
+      }}
+    >
+      <button
+        class="icon btn-flat"
+        aria-haspopup="true"
+        aria-expanded={{@active}}
+        href={{this.currentUser.path}}
+        aria-label={{concat
+          (or this.currentUser.name this.currentUser.username)
+          (i18n "user.account_possessive")
+        }}
+        data-auto-route="true"
+        {{on "click" this.click}}
+      >
+        <Notifications @active={{@active}} />
+      </button>
+    </li>
+  </template>
+}
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-header/user-dropdown/notifications.gjs b/app/assets/javascripts/discourse/app/components/glimmer-header/user-dropdown/notifications.gjs
new file mode 100644
index 00000000000..572e932b3f7
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-header/user-dropdown/notifications.gjs
@@ -0,0 +1,122 @@
+import Component from "@glimmer/component";
+import { hash } from "@ember/helper";
+import { inject as service } from "@ember/service";
+import { htmlSafe } from "@ember/template";
+import {
+  addExtraUserClasses,
+  renderAvatar,
+} from "discourse/helpers/user-avatar";
+import icon from "discourse-common/helpers/d-icon";
+import i18n from "discourse-common/helpers/i18n";
+import UserTip from "../../user-tip";
+import UserStatusBubble from "./user-status-bubble";
+
+export default class Notifications extends Component {
+  @service currentUser;
+  @service siteSettings;
+
+  avatarSize = "medium";
+
+  get avatar() {
+    let avatarAttrs = {};
+    addExtraUserClasses(this.currentUser, avatarAttrs);
+    return htmlSafe(
+      renderAvatar(this.currentUser, {
+        imageSize: this.avatarSize,
+        alt: "user.avatar.header_title",
+        template: this.currentUser.avatar_template,
+        username: this.currentUser.username,
+        name: this.siteSettings.enable_names && this.currentUser.name,
+        ...avatarAttrs,
+      })
+    );
+  }
+
+  get _shouldHighlightAvatar() {
+    return (
+      !this.currentUser.read_first_notification &&
+      !this.currentUser.enforcedSecondFactor &&
+      !this.args.active
+    );
+  }
+
+  get isInDoNotDisturb() {
+    return this.currentUser.isInDoNotDisturb();
+  }
+
+  <template>
+    {{this.avatar}}
+
+    {{#if this._shouldHighlightAvatar}}
+      <UserTip
+        @id="first_notification"
+        @triggerSelector=".header-dropdown-toggle.current-user"
+        @placement="bottom-end"
+        @titleText={{i18n "user_tips.first_notification.title"}}
+        @contentText={{i18n "user_tips.first_notification.content"}}
+      />
+    {{/if}}
+
+    {{#if this.currentUser.status}}
+      <UserStatusBubble
+        @timezone={{this.this.currentUser.user_option.timezone}}
+        @status={{this.currentUser.status}}
+      />
+    {{/if}}
+
+    {{#if this.isInDoNotDisturb}}
+      <div class="do-not-disturb-background">{{icon "moon"}}</div>
+    {{else}}
+      {{#if this.currentUser.new_personal_messages_notifications_count}}
+        <a
+          href="#"
+          class="badge-notification with-icon new-pms"
+          title={{i18n
+            "notifications.tooltip.new_message_notification"
+            (hash
+              count=this.currentUser.new_personal_messages_notifications_count
+            )
+          }}
+          aria-label={{i18n
+            "notifications.tooltip.new_message_notification"
+            (hash
+              count=this.currentUser.new_personal_messages_notifications_count
+            )
+          }}
+        >
+          {{icon "envelope"}}
+        </a>
+      {{else if this.currentUser.unseen_reviewable_count}}
+        <a
+          href="#"
+          class="badge-notification with-icon new-reviewables"
+          title={{i18n
+            "notifications.tooltip.new_reviewable"
+            (hash count=this.currentUser.unseen_reviewable_count)
+          }}
+          aria-label={{i18n
+            "notifications.tooltip.new_reviewable"
+            (hash count=this.currentUser.unseen_reviewable_count)
+          }}
+        >
+          {{icon "flag"}}
+        </a>
+      {{else if this.currentUser.all_unread_notifications_count}}
+        <a
+          href="#"
+          class="badge-notification unread-notifications"
+          title={{i18n
+            "notifications.tooltip.regular"
+            (hash count=this.currentUser.all_unread_notifications_count)
+          }}
+          aria-label={{i18n
+            "user.notifications"
+            (hash count=this.currentUser.all_unread_notifications_count)
+          }}
+        >
+          {{this.currentUser.all_unread_notifications_count}}
+        </a>
+      {{/if}}
+    {{/if}}
+  </template>
+}
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-header/user-dropdown/user-status-bubble.gjs b/app/assets/javascripts/discourse/app/components/glimmer-header/user-dropdown/user-status-bubble.gjs
new file mode 100644
index 00000000000..46cc93e595d
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-header/user-dropdown/user-status-bubble.gjs
@@ -0,0 +1,25 @@
+import { hash } from "@ember/helper";
+import emoji from "discourse/helpers/emoji";
+import I18n from "discourse-i18n";
+
+const title = (description, endsAt, timezone) => {
+  let content = description;
+  if (endsAt) {
+    const until = moment
+      .tz(endsAt, timezone)
+      .format(I18n.t("dates.long_date_without_year"));
+    content += `\n${I18n.t("until")} ${until}`;
+  }
+  return content;
+};
+
+const UserStatusBubble = <template>
+  <div class="user-status-background">
+    {{emoji
+      @status.emoji
+      (hash title=(title @status.description @status.ends_at @timezone))
+    }}
+  </div>
+</template>;
+
+export default UserStatusBubble;
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-header/user-menu-wrapper.gjs b/app/assets/javascripts/discourse/app/components/glimmer-header/user-menu-wrapper.gjs
new file mode 100644
index 00000000000..0bd90b20202
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-header/user-menu-wrapper.gjs
@@ -0,0 +1,60 @@
+import Component from "@glimmer/component";
+import { hash } from "@ember/helper";
+import { action } from "@ember/object";
+import { prefersReducedMotion } from "discourse/lib/utilities";
+import { isTesting } from "discourse-common/config/environment";
+import discourseLater from "discourse-common/lib/later";
+import closeOnClickOutside from "../../modifiers/close-on-click-outside";
+import UserMenu from "../user-menu/menu";
+
+export default class UserMenuWrapper extends Component {
+  @action
+  clickOutside(e) {
+    if (
+      e.target.classList.contains("header-cloak") &&
+      !prefersReducedMotion()
+    ) {
+      const panel = document.querySelector(".menu-panel");
+      const headerCloak = document.querySelector(".header-cloak");
+      const finishPosition =
+        document.documentElement.classList["direction"] === "rtl"
+          ? "-340px"
+          : "340px";
+      panel
+        .animate([{ transform: `translate3d(${finishPosition}, 0, 0)` }], {
+          duration: 200,
+          fill: "forwards",
+          easing: "ease-in",
+        })
+        .finished.then(() => {
+          if (isTesting()) {
+            this.args.toggleUserMenu();
+          } else {
+            discourseLater(() => this.args.toggleUserMenu());
+          }
+        });
+      headerCloak.animate([{ opacity: 0 }], {
+        duration: 200,
+        fill: "forwards",
+        easing: "ease-in",
+      });
+    } else {
+      this.args.toggleUserMenu();
+    }
+  }
+
+  <template>
+    <div
+      class="user-menu-dropdown-wrapper"
+      {{closeOnClickOutside
+        this.clickOutside
+        (hash
+          targetSelector=".user-menu-panel"
+          secondaryTargetSelector=".user-menu-panel"
+        )
+      }}
+    >
+      <UserMenu @closeUserMenu={{@toggleUserMenu}} />
+    </div>
+  </template>
+}
diff --git a/app/assets/javascripts/discourse/app/components/glimmer-site-header.gjs b/app/assets/javascripts/discourse/app/components/glimmer-site-header.gjs
new file mode 100644
index 00000000000..4dc239515e1
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/glimmer-site-header.gjs
@@ -0,0 +1,502 @@
+import Component from "@glimmer/component";
+import { DEBUG } from "@glimmer/env";
+import { action } from "@ember/object";
+import didInsert from "@ember/render-modifiers/modifiers/did-insert";
+import { cancel, schedule } from "@ember/runloop";
+import { inject as service } from "@ember/service";
+import { waitForPromise } from "@ember/test-waiters";
+import ItsATrap from "@discourse/itsatrap";
+import concatClass from "discourse/helpers/concat-class";
+import scrollLock from "discourse/lib/scroll-lock";
+import SwipeEvents from "discourse/lib/swipe-events";
+import { isTesting } from "discourse-common/config/environment";
+import discourseLater from "discourse-common/lib/later";
+import { bind, debounce } from "discourse-common/utils/decorators";
+import GlimmerHeader from "./glimmer-header";
+
+let _menuPanelClassesToForceDropdown = [];
+const PANEL_WIDTH = 340;
+
+export default class GlimmerSiteHeader extends Component {
+  @service appEvents;
+  @service currentUser;
+  @service site;
+  @service header;
+
+  pxClosed;
+  headerElement;
+  docking;
+  _dockedHeader = false;
+  _animate = false;
+  _headerWrap;
+  _swipeMenuOrigin;
+  _swipeEvents;
+  _applicationElement;
+  _resizeObserver;
+  _docAt;
+
+  constructor() {
+    super(...arguments);
+    this.docking = new Docking(this.dockCheck);
+
+    if (this.currentUser?.staff) {
+      document.body.classList.add("staff");
+    }
+
+    schedule("afterRender", () => this.animateMenu());
+  }
+
+  get dropDownHeaderEnabled() {
+    return !this.sidebarEnabled || this.site.narrowDesktopView;
+  }
+
+  get leftMenuClass() {
+    if (document.querySelector("html").classList["direction"] === "rtl") {
+      return "user-menu";
+    } else {
+      return "hamburger-panel";
+    }
+  }
+
+  @bind
+  updateHeaderOffset() {
+    // Safari likes overscolling the page (on both iOS and macOS).
+    // This shows up as a negative value in window.scrollY.
+    // We can use this to offset the headerWrap's top offset to avoid
+    // jitteriness and bad positioning.
+    const windowOverscroll = Math.min(0, window.scrollY);
+
+    // The headerWrap's top offset can also be a negative value on Safari,
+    // because of the changing height of the viewport (due to the URL bar).
+    // For our use case, it's best to ensure this is clamped to 0.
+    const headerWrapTop = Math.max(
+      0,
+      Math.floor(this._headerWrap.getBoundingClientRect().top)
+    );
+    let offsetTop = headerWrapTop + windowOverscroll;
+
+    if (DEBUG && isTesting()) {
+      offsetTop -= document
+        .getElementById("ember-testing-container")
+        .getBoundingClientRect().top;
+
+      offsetTop -= 1; // For 1px border on testing container
+    }
+
+    const documentStyle = document.documentElement.style;
+    const currentValue =
+      parseInt(documentStyle.getPropertyValue("--header-offset"), 10) || 0;
+    const newValue = this._headerWrap.offsetHeight + offsetTop;
+    if (currentValue !== newValue) {
+      documentStyle.setProperty("--header-offset", `${newValue}px`);
+    }
+  }
+
+  @bind
+  _onScroll() {
+    schedule("afterRender", this.updateHeaderOffset);
+  }
+
+  @action
+  setupHeader() {
+    this.appEvents.on("user-menu:rendered", this, this.animateMenu);
+    if (this.dropDownHeaderEnabled) {
+      this.appEvents.on(
+        "sidebar-hamburger-dropdown:rendered",
+        this,
+        this.animateMenu
+      );
+    }
+
+    this._headerWrap = document.querySelector(".d-header-wrap");
+    if (this._headerWrap) {
+      schedule("afterRender", () => {
+        this.headerElement = this._headerWrap.querySelector("header.d-header");
+        this.updateHeaderOffset();
+        document.documentElement.style.setProperty(
+          "--header-top",
+          `${this.headerElement.offsetTop}px`
+        );
+      });
+
+      window.addEventListener("scroll", this._onScroll, {
+        passive: true,
+      });
+
+      this._itsatrap = new ItsATrap(this.headerElement);
+      const dirs = ["up", "down"];
+      this._itsatrap.bind(dirs, (e) => this._handleArrowKeysNav(e));
+
+      this._resizeObserver = new ResizeObserver((entries) => {
+        for (let entry of entries) {
+          if (entry.contentRect) {
+            const headerTop = this.headerElement?.offsetTop;
+            document.documentElement.style.setProperty(
+              "--header-top",
+              `${headerTop}px`
+            );
+            this.updateHeaderOffset();
+          }
+        }
+      });
+
+      this._resizeObserver.observe(this._headerWrap);
+
+      this._swipeEvents = new SwipeEvents(this._headerWrap);
+      if (this.site.mobileView) {
+        this._swipeEvents.addTouchListeners();
+        this._headerWrap.addEventListener("swipestart", this.onSwipeStart);
+        this._headerWrap.addEventListener("swipeend", this.onSwipeEnd);
+        this._headerWrap.addEventListener("swipecancel", this.onSwipeCancel);
+        this._headerWrap.addEventListener("swipe", this.onSwipe);
+      }
+    }
+  }
+
+  _handleArrowKeysNav(event) {
+    const activeTab = document.querySelector(
+      ".menu-tabs-container .btn.active"
+    );
+    if (activeTab) {
+      let activeTabNumber = Number(
+        document.activeElement.dataset.tabNumber || activeTab.dataset.tabNumber
+      );
+      const maxTabNumber =
+        document.querySelectorAll(".menu-tabs-container .btn").length - 1;
+      const isNext = event.key === "ArrowDown";
+      let nextTab = isNext ? activeTabNumber + 1 : activeTabNumber - 1;
+      if (isNext && nextTab > maxTabNumber) {
+        nextTab = 0;
+      }
+      if (!isNext && nextTab < 0) {
+        nextTab = maxTabNumber;
+      }
+      event.preventDefault();
+      document
+        .querySelector(
+          `.menu-tabs-container .btn[data-tab-number='${nextTab}']`
+        )
+        .focus();
+    }
+  }
+
+  @action
+  animateMenu() {
+    const menuPanels = document.querySelectorAll(".menu-panel");
+
+    if (menuPanels.length === 0) {
+      this._animate = this.site.mobileView || this.site.narrowDesktopView;
+      return;
+    }
+
+    let viewMode =
+      this.site.mobileView || this.site.narrowDesktopView
+        ? "slide-in"
+        : "drop-down";
+
+    menuPanels.forEach((panel) => {
+      if (menuPanelContainsClass(panel)) {
+        viewMode = "drop-down";
+        this._animate = false;
+      }
+
+      const cloakElement = document.querySelector(".header-cloak");
+
+      panel.classList.remove("drop-down");
+      panel.classList.remove("slide-in");
+      panel.classList.add(viewMode);
+
+      if (this._animate) {
+        let animationFinished = null;
+        let finalPosition = PANEL_WIDTH;
+        this._swipeMenuOrigin = "right";
+        if (
+          (this.site.mobileView || this.site.narrowDesktopView) &&
+          panel.parentElement.classList.contains(this.leftMenuClass)
+        ) {
+          this._swipeMenuOrigin = "left";
+          finalPosition = -PANEL_WIDTH;
+        }
+        animationFinished = panel.animate(
+          [{ transform: `translate3d(${finalPosition}px, 0, 0)` }],
+          {
+            fill: "forwards",
+          }
+        ).finished;
+
+        if (isTesting()) {
+          waitForPromise(animationFinished);
+        }
+
+        cloakElement.animate([{ opacity: 0 }], { fill: "forwards" });
+        cloakElement.style.display = "block";
+
+        animationFinished.then(() => {
+          if (isTesting()) {
+            this._animateOpening(panel);
+          } else {
+            discourseLater(() => this._animateOpening(panel));
+          }
+        });
+      }
+
+      this._animate = false;
+    });
+  }
+
+  @bind
+  dockCheck() {
+    if (this._docAt === null) {
+      if (!this.headerElement) {
+        return;
+      }
+      this._docAt = this.headerElement.offsetTop;
+    }
+
+    const main = (this._applicationElement ??=
+      document.querySelector(".ember-application"));
+    const offsetTop = main?.offsetTop ?? 0;
+    const offset = window.pageYOffset - offsetTop;
+    if (offset >= this._docAt) {
+      if (!this._dockedHeader) {
+        document.body.classList.add("docked");
+        this._dockedHeader = true;
+      }
+    } else {
+      if (this._dockedHeader) {
+        document.body.classList.remove("docked");
+        this._dockedHeader = false;
+      }
+    }
+  }
+
+  @bind
+  _animateOpening(panel, event = null) {
+    const cloakElement = document.querySelector(".header-cloak");
+    let durationMs = this._swipeEvents.getMaxAnimationTimeMs();
+    if (event && this.pxClosed > 0) {
+      durationMs = this._swipeEvents.getMaxAnimationTimeMs(
+        this.pxClosed / Math.abs(event.velocityX)
+      );
+    }
+    const timing = {
+      duration: durationMs > 0 ? durationMs : 0,
+      fill: "forwards",
+      easing: "ease-out",
+    };
+    panel.animate([{ transform: `translate3d(0, 0, 0)` }], timing);
+    cloakElement?.animate?.([{ opacity: 1 }], timing);
+    this.pxClosed = null;
+  }
+
+  @bind
+  _animateClosing(event, panel, menuOrigin) {
+    this._animate = true;
+    const cloakElement = document.querySelector(".header-cloak");
+    let durationMs = this._swipeEvents.getMaxAnimationTimeMs();
+    if (event && this.pxClosed > 0) {
+      const distancePx = PANEL_WIDTH - this.pxClosed;
+      durationMs = this._swipeEvents.getMaxAnimationTimeMs(
+        distancePx / Math.abs(event.velocityX)
+      );
+    }
+    const timing = {
+      duration: durationMs > 0 ? durationMs : 0,
+      fill: "forwards",
+    };
+
+    let endPosition = -PANEL_WIDTH; //origin left
+    if (menuOrigin === "right") {
+      endPosition = PANEL_WIDTH;
+    }
+    panel.animate(
+      [{ transform: `translate3d(${endPosition}px, 0, 0)` }],
+      timing
+    );
+    if (cloakElement) {
+      cloakElement.animate([{ opacity: 0 }], timing);
+      cloakElement.style.display = "none";
+
+      // to ensure that the cloak is cleared after animation we need to toggle any active menus
+      if (this.header.hamburgerVisible || this.header.userVisible) {
+        this.header.hamburgerVisible = false;
+        this.header.userVisible = false;
+      }
+    }
+    this.pxClosed = null;
+  }
+
+  @bind
+  onSwipeStart(event) {
+    const e = event.detail;
+    const center = e.center;
+    const swipeOverValidElement = document
+      .elementsFromPoint(center.x, center.y)
+      .some(
+        (ele) =>
+          ele.classList.contains("panel-body") ||
+          ele.classList.contains("header-cloak")
+      );
+    if (
+      swipeOverValidElement &&
+      (e.direction === "left" || e.direction === "right")
+    ) {
+      scrollLock(true, document.querySelector(".panel-body"));
+    } else {
+      event.preventDefault();
+    }
+  }
+
+  @bind
+  onSwipeEnd(event) {
+    const e = event.detail;
+    const menuPanels = document.querySelectorAll(".menu-panel");
+    scrollLock(false, document.querySelector(".panel-body"));
+    menuPanels.forEach((panel) => {
+      if (this._swipeEvents.shouldCloseMenu(e, this._swipeMenuOrigin)) {
+        this._animateClosing(e, panel, this._swipeMenuOrigin);
+      } else {
+        this._animateOpening(panel, e);
+      }
+    });
+  }
+
+  @bind
+  onSwipeCancel() {
+    const menuPanels = document.querySelectorAll(".menu-panel");
+    scrollLock(false, document.querySelector(".panel-body"));
+    menuPanels.forEach((panel) => {
+      this._animateOpening(panel);
+    });
+  }
+
+  @bind
+  onSwipe(event) {
+    const e = event.detail;
+
+    const movingElement = document.querySelector(".menu-panel");
+    const cloakElement = document.querySelector(".header-cloak");
+
+    //origin left
+    this.pxClosed = Math.max(0, -e.deltaX);
+    let translation = -this.pxClosed;
+    if (this._swipeMenuOrigin === "right") {
+      this.pxClosed = Math.max(0, e.deltaX);
+      translation = this.pxClosed;
+    }
+
+    movingElement.animate(
+      [{ transform: `translate3d(${translation}px, 0, 0)` }],
+      {
+        fill: "forwards",
+      }
+    );
+    cloakElement?.animate?.(
+      [
+        {
+          opacity: (PANEL_WIDTH - this.pxClosed) / PANEL_WIDTH,
+        },
+      ],
+      { fill: "forwards" }
+    );
+  }
+
+  willDestroy() {
+    super.willDestroy(...arguments);
+    this.docking.destroy();
+    this.appEvents.off("user-menu:rendered", this, this.animateMenu);
+
+    if (this.dropDownHeaderEnabled) {
+      this.appEvents.off(
+        "sidebar-hamburger-dropdown:rendered",
+        this,
+        this.animateMenu
+      );
+    }
+
+    this._itsatrap?.destroy();
+    this._itsatrap = null;
+
+    window.removeEventListener("scroll", this._onScroll);
+    this._resizeObserver?.disconnect();
+    if (this.site.mobileView) {
+      this._headerWrap.removeEventListener("swipestart", this.onSwipeStart);
+      this._headerWrap.removeEventListener("swipeend", this.onSwipeEnd);
+      this._headerWrap.removeEventListener("swipecancel", this.onSwipeCancel);
+      this._headerWrap.removeEventListener("swipe", this.onSwipe);
+      this._swipeEvents.removeTouchListeners();
+    }
+  }
+
+  <template>
+    <div
+      class={{concatClass
+        (unless this.site.mobileView "drop-down-mode")
+        "d-header-wrap"
+      }}
+      {{didInsert this.setupHeader}}
+    >
+      <GlimmerHeader
+        @canSignUp={{@canSignUp}}
+        @showSidebar={{@showSidebar}}
+        @sidebarEnabled={{@sidebarEnabled}}
+        @toggleSidebar={{@toggleSidebar}}
+        @showCreateAccount={{@showCreateAccount}}
+        @showLogin={{@showLogin}}
+        @animateMenu={{this.animateMenu}}
+      />
+    </div>
+  </template>
+}
+
+const INITIAL_DELAY_MS = 50;
+const DEBOUNCE_MS = 5;
+class Docking {
+  dockCheck = null;
+  _initialTimer = null;
+  _queuedTimer = null;
+
+  constructor(dockCheck) {
+    this.dockCheck = dockCheck;
+    window.addEventListener("scroll", this.queueDockCheck, { passive: true });
+
+    // dockCheck might happen too early on full page refresh
+    this._initialTimer = discourseLater(this, this.dockCheck, INITIAL_DELAY_MS);
+  }
+
+  @debounce(DEBOUNCE_MS)
+  queueDockCheck() {
+    this._queuedTimer = this.dockCheck;
+  }
+
+  @action
+  destroy() {
+    if (this._queuedTimer) {
+      cancel(this._queuedTimer);
+    }
+
+    cancel(this._initialTimer);
+    window.removeEventListener("scroll", this.queueDockCheck);
+  }
+}
+
+function menuPanelContainsClass(menuPanel) {
+  if (!_menuPanelClassesToForceDropdown) {
+    return false;
+  }
+
+  for (let className of _menuPanelClassesToForceDropdown) {
+    if (menuPanel.classList.contains(className)) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+export function forceDropdownForMenuPanels(classNames) {
+  if (typeof classNames === "string") {
+    classNames = [classNames];
+  }
+  return _menuPanelClassesToForceDropdown.push(...classNames);
+}
diff --git a/app/assets/javascripts/discourse/app/components/legacy-header-icon-shim.gjs b/app/assets/javascripts/discourse/app/components/legacy-header-icon-shim.gjs
new file mode 100644
index 00000000000..f41426e00de
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/components/legacy-header-icon-shim.gjs
@@ -0,0 +1,25 @@
+import Component from "@glimmer/component";
+import { tracked } from "@glimmer/tracking";
+import { schedule } from "@ember/runloop";
+import { PANEL_WRAPPER_ID } from "discourse/widgets/header";
+import PanelPortal from "./glimmer-header/panel-portal";
+
+export default class LegacyHeaderIconShim extends Component {
+  @tracked panelElement;
+
+  constructor() {
+    super(...arguments);
+    schedule("afterRender", () => {
+      this.panelElement = document.querySelector(`#${PANEL_WRAPPER_ID}`);
+    });
+  }
+
+  <template>
+    {{#let
+      (component PanelPortal panelElement=this.panelElement)
+      as |panelPortal|
+    }}
+      <@component @panelPortal={{panelPortal}} />
+    {{/let}}
+  </template>
+}
diff --git a/app/assets/javascripts/discourse/app/components/menu-panel.hbs b/app/assets/javascripts/discourse/app/components/menu-panel.hbs
index e45928f73cb..2bd6d99bf21 100644
--- a/app/assets/javascripts/discourse/app/components/menu-panel.hbs
+++ b/app/assets/javascripts/discourse/app/components/menu-panel.hbs
@@ -1,5 +1,5 @@
 <div
-  class={{concat-class "search-menu-panel menu-panel" @animationClass}}
+  class={{concat-class "menu-panel" @panelClass @animationClass}}
   data-max-width="500"
 >
   <div class="panel-body">
diff --git a/app/assets/javascripts/discourse/app/components/search-menu-panel.hbs b/app/assets/javascripts/discourse/app/components/search-menu-panel.hbs
index e5009a35c21..970f67b0462 100644
--- a/app/assets/javascripts/discourse/app/components/search-menu-panel.hbs
+++ b/app/assets/javascripts/discourse/app/components/search-menu-panel.hbs
@@ -1,4 +1,7 @@
-<MenuPanel @animationClass={{this.animationClass}}>
+<MenuPanel
+  @animationClass={{this.animationClass}}
+  @panelClass="search-menu-panel"
+>
   <SearchMenu
     @onClose={{@closeSearchMenu}}
     @inlineResults={{true}}
diff --git a/app/assets/javascripts/discourse/app/components/search-menu.hbs b/app/assets/javascripts/discourse/app/components/search-menu.hbs
index c10cfe633c2..b13103587f0 100644
--- a/app/assets/javascripts/discourse/app/components/search-menu.hbs
+++ b/app/assets/javascripts/discourse/app/components/search-menu.hbs
@@ -67,7 +67,7 @@
       @clearSearch={{this.clearSearch}}
     />
   {{else if this.displayMenuPanelResults}}
-    <MenuPanel>
+    <MenuPanel @panelClass="search-menu-panel">
       <SearchMenu::Results
         @loading={{this.loading}}
         @noResults={{this.noResults}}
diff --git a/app/assets/javascripts/discourse/app/components/user-menu/items-list.hbs b/app/assets/javascripts/discourse/app/components/user-menu/items-list.hbs
index f8f32867b77..f0af1b0b1f2 100644
--- a/app/assets/javascripts/discourse/app/components/user-menu/items-list.hbs
+++ b/app/assets/javascripts/discourse/app/components/user-menu/items-list.hbs
@@ -24,6 +24,7 @@
         class="btn btn-default notifications-dismiss btn-icon-text"
         title={{this.dismissTitle}}
         {{on "click" this.dismissButtonClick}}
+        {{auto-focus}}
       >
         {{d-icon "check"}}
         {{i18n "user.dismiss"}}
diff --git a/app/assets/javascripts/discourse/app/helpers/bound-avatar-template.js b/app/assets/javascripts/discourse/app/helpers/bound-avatar-template.js
index 1454953e7fa..051844d8987 100644
--- a/app/assets/javascripts/discourse/app/helpers/bound-avatar-template.js
+++ b/app/assets/javascripts/discourse/app/helpers/bound-avatar-template.js
@@ -2,10 +2,10 @@ import { htmlSafe } from "@ember/template";
 import { isEmpty } from "@ember/utils";
 import { avatarImg } from "discourse-common/lib/avatar-utils";
 
-export default function boundAvatarTemplate(avatarTemplate, size) {
+export default function boundAvatarTemplate(avatarTemplate, size, options) {
   if (isEmpty(avatarTemplate)) {
     return htmlSafe("<div class='avatar-placeholder'></div>");
   } else {
-    return htmlSafe(avatarImg({ size, avatarTemplate }));
+    return htmlSafe(avatarImg({ size, avatarTemplate, ...options }));
   }
 }
diff --git a/app/assets/javascripts/discourse/app/lib/plugin-api.js b/app/assets/javascripts/discourse/app/lib/plugin-api.js
index 8c61ed1ffdf..f779ec63c66 100644
--- a/app/assets/javascripts/discourse/app/lib/plugin-api.js
+++ b/app/assets/javascripts/discourse/app/lib/plugin-api.js
@@ -9,6 +9,9 @@ import {
 import { addPluginDocumentTitleCounter } from "discourse/components/d-document";
 import { addToolbarCallback } from "discourse/components/d-editor";
 import { addCategorySortCriteria } from "discourse/components/edit-category-settings";
+import { addCustomHeaderClass } from "discourse/components/glimmer-header";
+import { addToHeaderIcons as addToGlimmerHeaderIcons } from "discourse/components/glimmer-header/icons";
+import { forceDropdownForMenuPanels as glimmerForceDropdownForMenuPanels } from "discourse/components/glimmer-site-header";
 import { addGlobalNotice } from "discourse/components/global-notice";
 import { _addBulkButton } from "discourse/components/modal/topic-bulk-actions";
 import { addWidgetCleanCallback } from "discourse/components/mount-widget";
@@ -142,7 +145,7 @@ import { modifySelectKit } from "select-kit/mixins/plugin-api";
 // docs/CHANGELOG-JAVASCRIPT-PLUGIN-API.md whenever you change the version
 // using the format described at https://keepachangelog.com/en/1.0.0/.
 
-export const PLUGIN_API_VERSION = "1.26.0";
+export const PLUGIN_API_VERSION = "1.27.0";
 
 // This helper prevents us from applying the same `modifyClass` over and over in test mode.
 function canModify(klass, type, resolverName, changes) {
@@ -931,6 +934,10 @@ class PluginApi {
    *
    **/
   addHeaderPanel(name, toggle, transformAttrs) {
+    // deprecated(
+    //   "addHeaderPanel has been removed. Use api.addToHeaderIcons instead.",
+    //   { id: "discourse.add-header-panel" }
+    // );
     attachAdditionalPanel(name, toggle, transformAttrs);
   }
 
@@ -1775,17 +1782,36 @@ class PluginApi {
     addExtraIconRenderer(renderer);
   }
   /**
-   * Adds a widget to the header-icon ul. The widget must already be created. You can create new widgets
+   * Adds a widget or a component to the header-icon ul.
+   *
+   * If adding a widget it must already be created. You can create new widgets
    * in a theme or plugin via an initializer prior to calling this function.
    *
    * ```
    * api.addToHeaderIcons(
-   *  createWidget('some-widget')
+   *  createWidget("some-widget")
+   * ```
+   *
+   * If adding a component you can pass the component directly. Additionally, you can
+   * utilize the `@panelPortal` argument to create a dropdown panel. This can be useful when
+   * you want create a button in the header that opens a dropdown panel with additional content.
+   *
+   * ```
+   * api.addToHeaderIcons(
+      <template>
+        <span>Icon</span>
+
+        <@panelPortal>
+          <div>Panel</div>
+        </@panelPortal>
+      </template>
+    );
    * ```
    *
    **/
   addToHeaderIcons(icon) {
     addToHeaderIcons(icon);
+    addToGlimmerHeaderIcons(icon);
   }
 
   /**
@@ -1965,6 +1991,7 @@ class PluginApi {
    */
   forceDropdownForMenuPanels(classNames) {
     forceDropdownForMenuPanels(classNames);
+    glimmerForceDropdownForMenuPanels(classNames);
   }
 
   /**
@@ -2775,6 +2802,19 @@ class PluginApi {
     addImageWrapperButton(label, btnClass, icon);
     addApiImageWrapperButtonClickEvent(fn);
   }
+
+  /**
+   * Add a custom css class to the header. The class or classes will live alongside the `d-header` class.
+   *
+   * ```
+   * api.addCustomHeaderClass("class-one");
+   * api.addCustomHeaderClass("class-two");
+   *
+   */
+
+  addCustomHeaderClass(klass) {
+    addCustomHeaderClass(klass);
+  }
 }
 
 // from http://stackoverflow.com/questions/6832596/how-to-compare-software-version-number-using-js-only-number
diff --git a/app/assets/javascripts/discourse/app/lib/render-topic-featured-link.js b/app/assets/javascripts/discourse/app/lib/render-topic-featured-link.js
index e7e368da123..28daf19f021 100644
--- a/app/assets/javascripts/discourse/app/lib/render-topic-featured-link.js
+++ b/app/assets/javascripts/discourse/app/lib/render-topic-featured-link.js
@@ -54,6 +54,7 @@ export default function renderTopicFeaturedLink(topic) {
     return "";
   }
 }
+// deprecated per components/glimmer-header/topic/featured-link.gjs
 export function topicFeaturedLinkNode(topic) {
   const meta = extractLinkMeta(topic);
   if (meta) {
diff --git a/app/assets/javascripts/discourse/app/mixins/docking.js b/app/assets/javascripts/discourse/app/mixins/docking.js
index a65e03bee0a..de39ae3a8b1 100644
--- a/app/assets/javascripts/discourse/app/mixins/docking.js
+++ b/app/assets/javascripts/discourse/app/mixins/docking.js
@@ -1,3 +1,4 @@
+// Deprecated in favor of app/assets/javascripts/discourse/app/services/docking.js
 import Mixin from "@ember/object/mixin";
 import { cancel } from "@ember/runloop";
 import discourseDebounce from "discourse-common/lib/debounce";
diff --git a/app/assets/javascripts/discourse/app/modifiers/close-on-click-outside.js b/app/assets/javascripts/discourse/app/modifiers/close-on-click-outside.js
new file mode 100644
index 00000000000..6de6e2c1383
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/modifiers/close-on-click-outside.js
@@ -0,0 +1,44 @@
+import { registerDestructor } from "@ember/destroyable";
+import Modifier from "ember-modifier";
+import { bind } from "discourse-common/utils/decorators";
+
+export default class CloseOnClickOutside extends Modifier {
+  constructor(owner, args) {
+    super(owner, args);
+    registerDestructor(this, (instance) => instance.cleanup());
+  }
+
+  modify(element, [closeFn, { targetSelector, secondaryTargetSelector }]) {
+    this.closeFn = closeFn;
+    this.element = element;
+    this.targetSelector = targetSelector;
+    this.secondaryTargetSelector = secondaryTargetSelector;
+
+    document.addEventListener("pointerdown", this.check, {
+      passive: true,
+    });
+  }
+
+  @bind
+  check(event) {
+    if (this.element.contains(event.target)) {
+      return;
+    }
+
+    if (
+      document.querySelector(this.targetSelector).contains(event.target) ||
+      (this.secondaryTargetSelector &&
+        document
+          .querySelector(this.secondaryTargetSelector)
+          ?.contains(event.target))
+    ) {
+      return;
+    }
+
+    this.closeFn(event);
+  }
+
+  cleanup() {
+    document.removeEventListener("pointerdown", this.check);
+  }
+}
diff --git a/app/assets/javascripts/discourse/app/services/header.js b/app/assets/javascripts/discourse/app/services/header.js
new file mode 100644
index 00000000000..37039cd4eaf
--- /dev/null
+++ b/app/assets/javascripts/discourse/app/services/header.js
@@ -0,0 +1,10 @@
+import { tracked } from "@glimmer/tracking";
+import Service from "@ember/service";
+import { disableImplicitInjections } from "discourse/lib/implicit-injections";
+
+@disableImplicitInjections
+export default class Header extends Service {
+  @tracked topic = null;
+  @tracked hamburgerVisible = false;
+  @tracked userVisible = false;
+}
diff --git a/app/assets/javascripts/discourse/app/templates/application.hbs b/app/assets/javascripts/discourse/app/templates/application.hbs
index 5fb69275a9d..765aba5d480 100644
--- a/app/assets/javascripts/discourse/app/templates/application.hbs
+++ b/app/assets/javascripts/discourse/app/templates/application.hbs
@@ -9,18 +9,32 @@
   />
 
   {{#if this.showSiteHeader}}
-    <SiteHeader
-      @canSignUp={{this.canSignUp}}
-      @showCreateAccount={{route-action "showCreateAccount"}}
-      @showLogin={{route-action "showLogin"}}
-      @showKeyboard={{route-action "showKeyboardShortcutsHelp"}}
-      @toggleMobileView={{route-action "toggleMobileView"}}
-      @logout={{route-action "logout"}}
-      @sidebarEnabled={{this.sidebarEnabled}}
-      @navigationMenuQueryParamOverride={{this.navigationMenuQueryParamOverride}}
-      @showSidebar={{this.showSidebar}}
-      @toggleSidebar={{action "toggleSidebar"}}
-    />
+    {{#if this.currentUser.glimmer_header_enabled}}
+      <GlimmerSiteHeader
+        @canSignUp={{this.canSignUp}}
+        @showCreateAccount={{route-action "showCreateAccount"}}
+        @showLogin={{route-action "showLogin"}}
+        @showKeyboard={{route-action "showKeyboardShortcutsHelp"}}
+        @toggleMobileView={{route-action "toggleMobileView"}}
+        @logout={{route-action "logout"}}
+        @sidebarEnabled={{this.sidebarEnabled}}
+        @showSidebar={{this.showSidebar}}
+        @toggleSidebar={{this.toggleSidebar}}
+      />
+    {{else}}
+      <SiteHeader
+        @canSignUp={{this.canSignUp}}
+        @showCreateAccount={{route-action "showCreateAccount"}}
+        @showLogin={{route-action "showLogin"}}
+        @showKeyboard={{route-action "showKeyboardShortcutsHelp"}}
+        @toggleMobileView={{route-action "toggleMobileView"}}
+        @logout={{route-action "logout"}}
+        @sidebarEnabled={{this.sidebarEnabled}}
+        @navigationMenuQueryParamOverride={{this.navigationMenuQueryParamOverride}}
+        @showSidebar={{this.showSidebar}}
+        @toggleSidebar={{action "toggleSidebar"}}
+      />
+    {{/if}}
   {{/if}}
 
   <SoftwareUpdatePrompt />
@@ -36,7 +50,6 @@
   />
 
   <div id="main-outlet-wrapper" class="wrap" role="main">
-
     <div class="sidebar-wrapper">
       {{! empty div allows for animation }}
       {{#if (and this.sidebarEnabled this.showSidebar)}}
@@ -74,7 +87,6 @@
     </div>
 
     <PluginOutlet @name="after-main-outlet" />
-
   </div>
 
   <PluginOutlet
diff --git a/app/assets/javascripts/discourse/app/widgets/header.js b/app/assets/javascripts/discourse/app/widgets/header.js
index 6ba2d2b15cc..22c90ebdc7b 100644
--- a/app/assets/javascripts/discourse/app/widgets/header.js
+++ b/app/assets/javascripts/discourse/app/widgets/header.js
@@ -9,7 +9,9 @@ import { logSearchLinkClick } from "discourse/lib/search";
 import DiscourseURL from "discourse/lib/url";
 import { scrollTop } from "discourse/mixins/scroll-top";
 import { avatarImg } from "discourse/widgets/post";
-import RenderGlimmer from "discourse/widgets/render-glimmer";
+import RenderGlimmer, {
+  registerWidgetShim,
+} from "discourse/widgets/render-glimmer";
 import { createWidget } from "discourse/widgets/widget";
 import { isTesting } from "discourse-common/config/environment";
 import getURL from "discourse-common/lib/get-url";
@@ -18,6 +20,7 @@ import discourseLater from "discourse-common/lib/later";
 import I18n from "discourse-i18n";
 
 const SEARCH_BUTTON_ID = "search-button";
+export const PANEL_WRAPPER_ID = "additional-panel-wrapper";
 
 let _extraHeaderIcons = [];
 
@@ -231,6 +234,14 @@ createWidget("header-icons", {
   services: ["search"],
   tagName: "ul.icons.d-header-icons",
 
+  init() {
+    registerWidgetShim(
+      "extra-icon",
+      "div.wrapper",
+      hbs`<LegacyHeaderIconShim @component={{@data.component}} />`
+    );
+  },
+
   html(attrs) {
     if (this.siteSettings.login_required && !this.currentUser) {
       return [];
@@ -240,7 +251,11 @@ createWidget("header-icons", {
 
     if (_extraHeaderIcons) {
       _extraHeaderIcons.forEach((icon) => {
-        icons.push(this.attach(icon));
+        if (typeof icon === "string") {
+          icons.push(this.attach(icon));
+        } else {
+          icons.push(this.attach("extra-icon", { component: icon }));
+        }
       });
     }
 
@@ -542,6 +557,8 @@ export default createWidget("header", {
         }
       });
 
+      panels.push(h(`div#${PANEL_WRAPPER_ID}`));
+
       if (this.site.mobileView || this.site.narrowDesktopView) {
         panels.push(this.attach("header-cloak"));
       }
diff --git a/app/assets/javascripts/discourse/app/widgets/sidebar-toggle.js b/app/assets/javascripts/discourse/app/widgets/sidebar-toggle.js
index ae4305c763a..172788bc0de 100644
--- a/app/assets/javascripts/discourse/app/widgets/sidebar-toggle.js
+++ b/app/assets/javascripts/discourse/app/widgets/sidebar-toggle.js
@@ -1,3 +1,4 @@
+// Deprecated in favor of app/assets/javascripts/discourse/app/components/glimmer-header/sidebar-toggle.gjs
 import { createWidget } from "discourse/widgets/widget";
 
 export default createWidget("sidebar-toggle", {
diff --git a/app/assets/javascripts/discourse/app/widgets/user-status-bubble.js b/app/assets/javascripts/discourse/app/widgets/user-status-bubble.js
index 86ac3ec9e47..5ea546abf29 100644
--- a/app/assets/javascripts/discourse/app/widgets/user-status-bubble.js
+++ b/app/assets/javascripts/discourse/app/widgets/user-status-bubble.js
@@ -1,3 +1,5 @@
+// deprecated in favor of app/components/glimmer-header/user-dropdown/user-status-bubble.gjs
+
 import { createWidget } from "discourse/widgets/widget";
 import I18n from "discourse-i18n";
 
diff --git a/app/assets/javascripts/discourse/tests/helpers/qunit-helpers.js b/app/assets/javascripts/discourse/tests/helpers/qunit-helpers.js
index f6fc00a081f..dc547ad412c 100644
--- a/app/assets/javascripts/discourse/tests/helpers/qunit-helpers.js
+++ b/app/assets/javascripts/discourse/tests/helpers/qunit-helpers.js
@@ -18,6 +18,7 @@ import {
   cleanUpComposerUploadPreProcessor,
 } from "discourse/components/composer-editor";
 import { clearToolbarCallbacks } from "discourse/components/d-editor";
+import { clearExtraHeaderIcons as clearExtraGlimmerHeaderIcons } from "discourse/components/glimmer-header/icons";
 import { clearBulkButtons } from "discourse/components/modal/topic-bulk-actions";
 import { resetWidgetCleanCallbacks } from "discourse/components/mount-widget";
 import { resetDecorators as resetPluginOutletDecorators } from "discourse/components/plugin-connector";
@@ -223,6 +224,7 @@ export function testCleanup(container, app) {
   clearToolbarCallbacks();
   resetNotificationTypeRenderers();
   resetSidebarPanels();
+  clearExtraGlimmerHeaderIcons();
   clearExtraHeaderIcons();
   resetOnKeyUpCallbacks();
   resetItemSelectCallbacks();
diff --git a/app/assets/javascripts/discourse/tests/integration/components/site-header-test.js b/app/assets/javascripts/discourse/tests/integration/components/site-header-test.js
index 104b1977b36..aad268db647 100644
--- a/app/assets/javascripts/discourse/tests/integration/components/site-header-test.js
+++ b/app/assets/javascripts/discourse/tests/integration/components/site-header-test.js
@@ -1,3 +1,5 @@
+// deprecated in favor of spec/system/header/site_header_spec.rb
+
 import {
   click,
   render,
diff --git a/app/assets/javascripts/discourse/tests/integration/components/widgets/header-test.js b/app/assets/javascripts/discourse/tests/integration/components/widgets/header-test.js
index f7b3a42582b..28a2c1161fe 100644
--- a/app/assets/javascripts/discourse/tests/integration/components/widgets/header-test.js
+++ b/app/assets/javascripts/discourse/tests/integration/components/widgets/header-test.js
@@ -1,3 +1,5 @@
+// deprecated in favor of spec/system/header/header_spec.rb
+
 import { click, render } from "@ember/test-helpers";
 import { hbs } from "ember-cli-htmlbars";
 import { module, test } from "qunit";
diff --git a/app/assets/stylesheets/common/base/header.scss b/app/assets/stylesheets/common/base/header.scss
index cadf92de6e9..34b27ec6679 100644
--- a/app/assets/stylesheets/common/base/header.scss
+++ b/app/assets/stylesheets/common/base/header.scss
@@ -439,3 +439,13 @@ $mobile-avatar-height: 1.532em;
     background: transparent;
   }
 }
+
+#additional-panel-wrapper {
+  position: absolute;
+  // positions are relative to the .d-header .panel div
+  top: 100%; // directly underneath .panel
+  right: -10px; // 10px to the right of .panel - adjust as needed
+  max-height: 80vh;
+  border-radius: var(--d-border-radius-large);
+  overflow: auto;
+}
diff --git a/app/assets/stylesheets/mobile/header.scss b/app/assets/stylesheets/mobile/header.scss
index e83acd1130e..5f1d5d71065 100644
--- a/app/assets/stylesheets/mobile/header.scss
+++ b/app/assets/stylesheets/mobile/header.scss
@@ -45,7 +45,7 @@
     }
   }
   // Fade in header avatar + icons if topic title is not visible in mobile header
-  .panel {
+  .panel .icons {
     animation: fadein 0.5s;
     @media (prefers-reduced-motion) {
       animation-duration: 0s;
diff --git a/app/models/user.rb b/app/models/user.rb
index 0326df16c34..7283a46d608 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -1816,6 +1816,10 @@ class User < ActiveRecord::Base
     in_any_groups?(SiteSetting.experimental_new_new_view_groups_map)
   end
 
+  def glimmer_header_enabled?
+    in_any_groups?(SiteSetting.experimental_glimmer_header_groups_map)
+  end
+
   def watched_precedence_over_muted
     if user_option.watched_precedence_over_muted.nil?
       SiteSetting.watched_precedence_over_muted
diff --git a/app/serializers/current_user_serializer.rb b/app/serializers/current_user_serializer.rb
index ab6d6a2460f..d6634e2bdb4 100644
--- a/app/serializers/current_user_serializer.rb
+++ b/app/serializers/current_user_serializer.rb
@@ -74,7 +74,8 @@ class CurrentUserSerializer < BasicUserSerializer
              :new_new_view_enabled?,
              :use_experimental_topic_bulk_actions?,
              :use_experimental_topic_bulk_actions?,
-             :use_admin_sidebar
+             :use_admin_sidebar,
+             :glimmer_header_enabled?
 
   delegate :user_stat, to: :object, private: true
   delegate :any_posts, :draft_count, :pending_posts_count, :read_faq?, to: :user_stat
diff --git a/config/locales/server.en.yml b/config/locales/server.en.yml
index c466f3d2185..da18d7136e1 100644
--- a/config/locales/server.en.yml
+++ b/config/locales/server.en.yml
@@ -2523,6 +2523,7 @@ en:
     enable_custom_sidebar_sections: "EXPERIMENTAL: Enable custom sidebar sections"
     experimental_topics_filter: "EXPERIMENTAL: Enables the experimental topics filter route at /filter"
     enable_experimental_lightbox: "EXPERIMENTAL: Replace the default image lightbox with the revamped design."
+    experimental_glimmer_header_groups: "EXPERIMENTAL: Render the site header as glimmer components."
 
     experimental_form_templates: "EXPERIMENTAL: Enable the form templates feature. <b>After enabled,</b> manage the templates at <a href='%{base_path}/admin/customize/form-templates'>Customize / Templates</a>."
     admin_sidebar_enabled_groups: "EXPERIMENTAL: Enable sidebar navigation for the admin UI for the specified groups, which replaces the top-level admin navigation buttons."
diff --git a/config/site_settings.yml b/config/site_settings.yml
index da0d658fc75..33273151576 100644
--- a/config/site_settings.yml
+++ b/config/site_settings.yml
@@ -2328,6 +2328,12 @@ developer:
   instrument_gc_stat_per_request:
     default: false
     hidden: true
+  experimental_glimmer_header_groups:
+    client: true
+    type: group_list
+    list_type: compact
+    default: ""
+    allow_any: false
   admin_sidebar_enabled_groups:
     type: group_list
     list_type: compact
diff --git a/docs/CHANGELOG-JAVASCRIPT-PLUGIN-API.md b/docs/CHANGELOG-JAVASCRIPT-PLUGIN-API.md
index 97ef5f3b1a0..5dba9560276 100644
--- a/docs/CHANGELOG-JAVASCRIPT-PLUGIN-API.md
+++ b/docs/CHANGELOG-JAVASCRIPT-PLUGIN-API.md
@@ -7,6 +7,10 @@ in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.27.0] - 2024-02-21
+
+- Updated `addToHeaderIcons` to take a component instead of just a widget (masked a string). Additionally, you can can now utilize the `@panelPortal` argument to create a dropdown panel. This can be useful when * you want create a button in the header that opens a dropdown panel with additional content.
+
 ## [1.26.0] - 2024-02-21
 
 - Added `renderBeforeWrapperOutlet` which is used for rendering components before the content of wrapper plugin outlets 
diff --git a/spec/system/header_spec.rb b/spec/system/header_spec.rb
new file mode 100644
index 00000000000..48d3e2a7e85
--- /dev/null
+++ b/spec/system/header_spec.rb
@@ -0,0 +1,211 @@
+# frozen_string_literal: true
+
+RSpec.describe "Glimmer Header", type: :system do
+  fab!(:current_user) { Fabricate(:user) }
+  before { SiteSetting.experimental_glimmer_header_groups = Group::AUTO_GROUPS[:everyone] }
+
+  it "renders basics" do
+    visit "/"
+    expect(page).to have_css("header.d-header")
+    expect(page).to have_css("#site-logo")
+  end
+
+  it "displays sign up / login buttons" do
+    visit "/"
+    expect(page).to have_css("button.sign-up-button")
+    expect(page).to have_css("button.login-button")
+
+    find("button.sign-up-button").click
+    expect(page).to have_css(".d-modal.create-account")
+
+    click_outside
+
+    find("button.login-button").click
+    expect(page).to have_css(".d-modal.login-modal")
+  end
+
+  it "shows login button when login required" do
+    SiteSetting.login_required = true
+
+    visit "/"
+    expect(page).to have_css("button.login-button")
+    expect(page).to have_css("button.sign-up-button")
+    expect(page).not_to have_css("#search-button")
+    expect(page).not_to have_css("button.btn-sidebar-toggle")
+  end
+
+  it "renders unread notifications count when user's notifications count is updated" do
+    Fabricate(
+      :notification,
+      user: current_user,
+      high_priority: true,
+      read: false,
+      created_at: 8.minutes.ago,
+    )
+
+    sign_in(current_user)
+    visit "/"
+    expect(page).to have_selector(
+      ".header-dropdown-toggle.current-user .unread-notifications",
+      text: "1",
+    )
+  end
+
+  it "doesn't show pending reviewables count for non-legacy navigation menu" do
+    SiteSetting.navigation_menu = "sidebar"
+    current_user.update!(admin: true)
+    Fabricate(:reviewable)
+
+    sign_in(current_user)
+    visit "/"
+    expect(page).not_to have_selector(".hamburger-dropdown .badge-notification")
+  end
+
+  it "closes revamped menu when clicking outside" do
+    sign_in(current_user)
+    visit "/"
+    find(".header-dropdown-toggle.current-user").click
+    expect(page).to have_selector(".user-menu.revamped")
+    find("header.d-header").click
+    expect(page).not_to have_selector(".user-menu.revamped")
+  end
+
+  it "sets header's height css property" do
+    sign_in(current_user)
+    visit "/"
+    resize_element(".d-header", 90)
+    wait_for(timeout: 100) { get_computed_style_value(".d-header", "--header-offset") == "90px" }
+    expect(get_computed_style_value(".d-header", "--header-offset")).to eq("90px")
+
+    resize_element(".d-header", 60)
+    wait_for(timeout: 100) { get_computed_style_value(".d-header", "--header-offset") == "60px" }
+    expect(get_computed_style_value(".d-header", "--header-offset")).to eq("60px")
+  end
+
+  it "moves focus between tabs using arrow keys" do
+    sign_in(current_user)
+    visit "/"
+    find(".header-dropdown-toggle.current-user").click
+    expect(active_element_id).to eq("user-menu-button-all-notifications")
+
+    find("##{active_element_id}").send_keys(:arrow_down)
+    expect(active_element_id).to eq("user-menu-button-replies")
+
+    4.times { find("##{active_element_id}").send_keys(:arrow_down) }
+    expect(active_element_id).to eq("user-menu-button-profile")
+
+    find("##{active_element_id}").send_keys(:arrow_down)
+    expect(active_element_id).to eq("user-menu-button-all-notifications")
+
+    find("##{active_element_id}").send_keys(:arrow_up)
+    expect(active_element_id).to eq("user-menu-button-profile")
+  end
+
+  it "prioritizes new personal messages bubble over unseen reviewables and regular notifications bubbles" do
+    Fabricate(:private_message_notification, user: current_user)
+    Fabricate(
+      :notification,
+      user: current_user,
+      high_priority: true,
+      read: false,
+      created_at: 8.minutes.ago,
+    )
+
+    sign_in(current_user)
+    visit "/"
+    expect(page).not_to have_selector(
+      ".header-dropdown-toggle.current-user .badge-notification.unread-notifications",
+    )
+    expect(page).not_to have_selector(
+      ".header-dropdown-toggle.current-user .badge-notification.with-icon.new-reviewables",
+    )
+
+    expect(page).to have_selector(
+      ".header-dropdown-toggle.current-user .badge-notification.with-icon.new-pms",
+    )
+    expect(page).to have_css(".d-icon-envelope")
+    expect(
+      find(".header-dropdown-toggle.current-user .badge-notification.with-icon.new-pms")[:title],
+    ).to eq(I18n.t("js.notifications.tooltip.new_message_notification", count: 1))
+  end
+
+  it "prioritizes unseen reviewables bubble over regular notifications" do
+    current_user.update!(admin: true)
+    Fabricate(:reviewable)
+
+    sign_in(current_user)
+    visit "/"
+    expect(page).not_to have_selector(
+      ".header-dropdown-toggle.current-user .badge-notification.unread-notifications",
+    )
+    expect(page).to have_selector(
+      ".header-dropdown-toggle.current-user .badge-notification.with-icon.new-reviewables",
+    )
+    expect(page).not_to have_selector(
+      ".header-dropdown-toggle.current-user .badge-notification.with-icon.new-pms",
+    )
+  end
+
+  it "shows regular notifications bubble if there are neither new personal messages nor unseen reviewables" do
+    Fabricate.times(
+      3,
+      :notification,
+      user: current_user,
+      high_priority: true,
+      read: false,
+      created_at: 8.minutes.ago,
+    )
+
+    sign_in(current_user)
+    visit "/"
+    expect(page).to have_selector(
+      ".header-dropdown-toggle.current-user .badge-notification.unread-notifications",
+      text: "3",
+    )
+    expect(
+      find(".header-dropdown-toggle.current-user .badge-notification.unread-notifications")[:title],
+    ).to eq(I18n.t("js.notifications.tooltip.regular", count: 3))
+    expect(page).not_to have_selector(
+      ".header-dropdown-toggle.current-user .badge-notification.with-icon.new-reviewables",
+    )
+    expect(page).not_to have_selector(
+      ".header-dropdown-toggle.current-user .badge-notification.with-icon.new-pms",
+    )
+  end
+
+  context "when logged in and login required" do
+    fab!(:current_user) { Fabricate(:user) }
+
+    it "displays current user when logged in and login required" do
+      SiteSetting.login_required = true
+      sign_in(current_user)
+
+      visit "/"
+      expect(page).not_to have_css("button.login-button")
+      expect(page).not_to have_css("button.sign-up-button")
+      expect(page).to have_css("#search-button")
+      expect(page).to have_css("button.btn-sidebar-toggle")
+      expect(page).to have_css("#current-user")
+    end
+  end
+
+  private
+
+  def get_computed_style_value(selector, property)
+    page.evaluate_script(
+      "window.getComputedStyle(document.querySelector('#{selector}')).getPropertyValue('#{property}')",
+    ).strip
+  end
+
+  def resize_element(selector, size)
+    page.evaluate_script("document.querySelector('#{selector}').style.height = '#{size}px'")
+  end
+
+  def active_element_id
+    page.evaluate_script("document.activeElement.id")
+  end
+
+  def click_outside
+    find(".d-modal").click(x: 0, y: 0)
+  end
+end