FIX: ensures tabbing from trigger focus content (#30064)

`DMenu` is using in-element, which means the content is detached from the trigger, and pressing tab from the trigger is not going to jump into the content. This commit catches the tab event and attempts to focus the first focusable element of the content.

app/assets/javascripts/float-kit/addon/components/d-menu.gjs

@@ -1,6 +1,9 @@
 import Component from "@glimmer/component";
 import { concat } from "@ember/helper";
+import { on } from "@ember/modifier";
+import { action } from "@ember/object";
 import { getOwner } from "@ember/owner";
+import didInsert from "@ember/render-modifiers/modifiers/did-insert";
 import { service } from "@ember/service";
 import { modifier } from "ember-modifier";
 import { and } from "truth-helpers";
@@ -31,6 +34,28 @@ export default class DMenu extends Component {
     };
   });
 
+  @action
+  registerFloatBody(element) {
+    this.body = element;
+  }
+
+  @action
+  forwardTabToContent(event) {
+    if (!this.body) {
+      return;
+    }
+
+    if (event.key === "Tab") {
+      event.preventDefault();
+
+      const firstFocusable = this.body.querySelector(
+        'button, a, input:not([type="hidden"]), select, textarea, [tabindex]:not([tabindex="-1"])'
+      );
+
+      firstFocusable?.focus() || this.body.focus();
+    }
+  }
+
   get menuId() {
     return `d-menu-${this.menuInstance.id}`;
   }
@@ -73,6 +98,7 @@ export default class DMenu extends Component {
       @translatedTitle={{@title}}
       @disabled={{@disabled}}
       aria-expanded={{if this.menuInstance.expanded "true" "false"}}
+      {{on "keydown" this.forwardTabToContent}}
       ...attributes
     >
       {{#if (has-block "trigger")}}
@@ -122,6 +148,7 @@ export default class DMenu extends Component {
           @innerClass="fk-d-menu__inner-content"
           @role="dialog"
           @inline={{this.options.inline}}
+          {{didInsert this.registerFloatBody}}
         >
           {{#if (has-block)}}
             {{yield this.componentArgs}}