UX: Allow the timeline to scroll mid post

app/assets/javascripts/discourse/components/scrolling-post-stream.js.es6

@@ -27,8 +27,9 @@ export default MountWidget.extend({
   widget: 'post-stream',
   _topVisible: null,
   _bottomVisible: null,
-  _currentPost: -1,
+  _currentPost: null,
   _currentVisible: null,
+  _currentPercent: null,
 
   buildArgs() {
     return this.getProperties('posts',
@@ -82,13 +83,20 @@ export default MountWidget.extend({
     if (windowBottom > bodyHeight) { windowBottom = bodyHeight; }
     if (viewportBottom > bodyHeight) { viewportBottom = bodyHeight; }
 
-    let currentPost = -1;
+    let currentPost = null;
+    let percent = null;
 
     const offset = offsetCalculator();
+    const topCheck = Math.ceil(windowTop + offset);
     if (windowTop < offset) {
       currentPost = 0;
+      const $post = $($posts[0]);
+      percent = windowTop > 0 ? (topCheck - $post.offset().top) / $post.height() : 0;
     }
 
+    // uncomment to debug the eyeline
+    // $('.debug-eyeline').css({ height: '1px', width: '100%', backgroundColor: 'blue', position: 'absolute', top: `${topCheck}px` });
+
     let bottomView = topView;
     while (bottomView < $posts.length) {
       const post = $posts[bottomView];
@@ -97,7 +105,8 @@ export default MountWidget.extend({
       if (!$post) { break; }
 
       const viewTop = $post.offset().top;
-      const viewBottom = viewTop + $post.height() + 100;
+      const postHeight = $post.height();
+      const viewBottom = viewTop + postHeight;
 
       if (viewTop > viewportBottom) { break; }
 
@@ -105,7 +114,8 @@ export default MountWidget.extend({
         onscreen.push(bottomView);
       }
 
-      if (currentPost === -1 && (viewTop >= windowTop + offset)) {
+      if (currentPost === null && (viewTop <= topCheck) && (viewBottom > topCheck)) {
+        percent = (topCheck - viewTop) / postHeight;
         currentPost = bottomView;
       }
 
@@ -153,10 +163,16 @@ export default MountWidget.extend({
         this.sendAction('currentPostChanged', { post });
       }
 
+      if (percent !== null && this._currentPercent !== percent) {
+        this._currentPercent = percent;
+        this.sendAction('currentPostScrolled', { percent });
+      }
+
     } else {
       this._topVisible = null;
       this._bottomVisible = null;
-      this._currentPost = -1;
+      this._currentPost = null;
+      this._currentPercent = null;
     }
 
     const onscreenPostNumbers = [];

app/assets/javascripts/discourse/components/topic-timeline.js.es6

@@ -49,11 +49,11 @@ export default MountWidget.extend(Docking, {
 
   didInsertElement() {
     this._super();
-    this.dispatch('topic:current-post-changed', 'timeline-scrollarea');
+    this.dispatch('topic:current-post-scrolled', 'timeline-scrollarea');
   },
 
   willDestroyElement() {
     this._super();
-    this.appEvents.off('topic:current-post-changed');
+    this.appEvents.off('topic:current-post-scrolled');
   }
 });

app/assets/javascripts/discourse/controllers/topic.js.es6

@@ -22,6 +22,7 @@ export default Ember.Controller.extend(SelectedPostsCount, BufferedContent, {
   enteredIndex: null,
   retrying: false,
   userTriggeredProgress: null,
+  _progressIndex: null,
 
   topicDelegated: [
     'toggleMultiSelect',
@@ -204,7 +205,15 @@ export default Ember.Controller.extend(SelectedPostsCount, BufferedContent, {
 
       const postStream = model.get('postStream');
 
-      this.appEvents.trigger('topic:current-post-changed', postStream.progressIndexOfPost(post));
+      this._progressIndex = postStream.progressIndexOfPost(post);
+      this.appEvents.trigger('topic:current-post-changed', this._progressIndex);
+    },
+
+    currentPostScrolled(event) {
+      this.appEvents.trigger('topic:current-post-scrolled', {
+        postNumber: this._progressIndex,
+        percent: event.percent
+      });
     },
 
     // Called the the topmost visible post on the page changes.

app/assets/javascripts/discourse/mixins/docking.js.es6

@@ -8,7 +8,7 @@ export default Ember.Mixin.create({
   init() {
     this._super();
     this.queueDockCheck = () => {
-      Ember.run.debounce(this, this.dockCheck, helper, 20);
+      Ember.run.debounce(this, this.dockCheck, helper, 5);
     };
   },
 

app/assets/javascripts/discourse/templates/topic.hbs

@@ -117,6 +117,7 @@
               showInvite="showInvite"
               topVisibleChanged="topVisibleChanged"
               currentPostChanged="currentPostChanged"
+              currentPostScrolled="currentPostScrolled"
               bottomVisibleChanged="bottomVisibleChanged"
               selectPost="toggledSelectedPost"
               selectReplies="toggledSelectedPostReplies"

app/assets/javascripts/discourse/widgets/topic-timeline.js.es6

@@ -96,7 +96,7 @@ createWidget('timeline-scrollarea', {
     const topic = attrs.topic;
     const postStream = topic.get('postStream');
     const total = postStream.get('filteredPostsCount');
-    let current = Math.round(total * percentage);
+    let current = Math.floor(total * percentage);
 
     if (current < 1) { current = 1; }
     if (current > total) { current = total; }
@@ -167,11 +167,11 @@ createWidget('timeline-scrollarea', {
     this.sendWidgetAction('jumpToIndex', position.current);
   },
 
-  topicCurrentPostChanged(postNumber) {
+  topicCurrentPostScrolled(event) {
+    const { postNumber, percent } = event;
+
     // If the post number didn't change keep our scroll position
-    if (postNumber !== this.state.scrolledPost) {
-      this.state.percentage = this._percentFor(this.attrs.topic, postNumber);
-    }
+    this.state.percentage = this._percentFor(this.attrs.topic, parseFloat(postNumber) + percent);
   },
 
   _percentFor(topic, postNumber) {

app/views/layouts/application.html.erb

@@ -42,6 +42,8 @@
   </head>
 
   <body>
+    <div class='debug-eyeline'></div>
+
     <noscript data-path="<%= request.env['PATH_INFO'] %>">
       <%= render partial: 'header' %>
       <div id="main-outlet" class="wrap">