UX: Docking back button on topic timeline

app/assets/javascripts/discourse/widgets/hooks.js.es6

@@ -56,12 +56,12 @@ let _dragging;
 const DRAG_NAME       = "mousemove.discourse-widget-drag";
 const DRAG_NAME_TOUCH = "touchmove.discourse-widget-drag";
 
-function cancelDrag() {
+function cancelDrag(e) {
   $('body').removeClass('widget-dragging');
   $(document).off(DRAG_NAME).off(DRAG_NAME_TOUCH);
 
   if (_dragging) {
-    if (_dragging.dragEnd) { _dragging.dragEnd(); }
+    if (_dragging.dragEnd) { _dragging.dragEnd(e); }
     _dragging = null;
   }
 }
@@ -70,7 +70,7 @@ WidgetClickHook.setupDocumentCallback = function() {
   if (_watchingDocument) { return; }
 
   $(document).on('mousedown.discource-widget-drag, touchstart.discourse-widget-drag', e => {
-    cancelDrag();
+    cancelDrag(e);
     const widget = findWidget(e.target, DRAG_ATTRIBUTE_NAME);
     if (widget) {
       e.preventDefault();
@@ -87,7 +87,7 @@ WidgetClickHook.setupDocumentCallback = function() {
     }
   });
 
-  $(document).on('mouseup.discourse-widget-drag, touchend.discourse-widget-drag', () => cancelDrag());
+  $(document).on('mouseup.discourse-widget-drag, touchend.discourse-widget-drag', e => cancelDrag(e));
 
   $(document).on('click.discourse-widget', e => {
     nodeCallback(e.target, CLICK_ATTRIBUTE_NAME, w => w.click(e));

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

@@ -12,6 +12,15 @@ function clamp(p, min=0.0, max=1.0) {
   return Math.max(Math.min(p, max), min);
 }
 
+function attachBackButton(widget) {
+  return widget.attach('button', {
+    className: 'btn btn-primary btn-small back-button',
+    label: 'topic.timeline.back',
+    title: 'topic.timeline.back_description',
+    action: 'goBack'
+  });
+}
+
 createWidget('timeline-last-read', {
   tagName: 'div.timeline-last-read',
 
@@ -22,20 +31,12 @@ createWidget('timeline-last-read', {
   html(attrs) {
     const result = [ iconNode('minus', { class: 'progress' }) ];
     if (attrs.showButton) {
-      result.push(this.attach('button', {
+      result.push(attachBackButton(this));
-        className: 'btn btn-primary btn-small',
-        label: 'topic.timeline.back',
-        title: 'topic.timeline.back_description',
-        action: 'goBack'
-      }));
     }
 
     return result;
   },
 
-  goBack() {
-    this.sendWidgetAction('jumpToPost', this.attrs.lastRead);
-  }
 });
 
 function timelineDate(date) {
@@ -61,6 +62,9 @@ createWidget('timeline-scroller', {
       contents.push(h('div.timeline-ago', timelineDate(date)));
     }
 
+    if (attrs.showDockedButton) {
+      contents.push(attachBackButton(this));
+    }
     let result = [ h('div.timeline-handle'), h('div.timeline-scroller-content', contents) ];
 
     if (attrs.fullScreen) {
@@ -74,9 +78,11 @@ createWidget('timeline-scroller', {
     this.sendWidgetAction('updatePercentage', e.pageY);
   },
 
-  dragEnd() {
+  dragEnd(e) {
+    if (!$(e.target).is('button')) {
       this.sendWidgetAction('commit');
     }
+  }
 });
 
 createWidget('timeline-padding', {
@@ -153,22 +159,36 @@ createWidget('timeline-scrollarea', {
     const before = SCROLLAREA_REMAINING * percentage;
     const after = (SCROLLAREA_HEIGHT - before) - SCROLLER_HEIGHT;
 
+    let showButton = false;
+    const hasBackPosition =
+      position.lastRead > 3 &&
+      Math.abs(position.lastRead - position.current) > 3 &&
+      (position.lastRead && position.lastRead !== position.total);
+
+    if (hasBackPosition) {
+      const lastReadTop = Math.round(position.lastReadPercentage * SCROLLAREA_HEIGHT);
+      showButton = ((lastReadTop < before - (SCROLLER_HEIGHT * 0.5)) ||
+                   (lastReadTop > (before + SCROLLER_HEIGHT))) &&
+                   (lastReadTop < (SCROLLAREA_HEIGHT - SCROLLER_HEIGHT));
+    }
+
     const result = [
       this.attach('timeline-padding', { height: before }),
-      this.attach('timeline-scroller', _.merge(position, {fullScreen: attrs.fullScreen})),
+      this.attach('timeline-scroller', _.merge(position, {
+        showDockedButton: hasBackPosition && !showButton,
+        fullScreen: attrs.fullScreen
+      })),
       this.attach('timeline-padding', { height: after })
     ];
 
-    if (position.lastRead && position.lastRead !== position.total) {
+    if (hasBackPosition) {
       const lastReadTop = Math.round(position.lastReadPercentage * SCROLLAREA_HEIGHT);
-      if (lastReadTop > (before + SCROLLER_HEIGHT * 0.5)) {
       result.push(this.attach('timeline-last-read', {
         top: lastReadTop,
         lastRead: position.lastRead,
-          showButton: (lastReadTop > (before + SCROLLER_HEIGHT)) && (lastReadTop < (SCROLLAREA_HEIGHT - SCROLLER_HEIGHT))
+        showButton
       }));
     }
-    }
 
     return result;
   },
@@ -196,6 +216,10 @@ createWidget('timeline-scrollarea', {
   _percentFor(topic, postIndex) {
     const total = topic.get('postStream.filteredPostsCount');
     return clamp(parseFloat(postIndex - 1.0) / total);
+  },
+
+  goBack() {
+    this.sendWidgetAction('jumpToPost', this.position().lastRead);
   }
 });
 

app/assets/stylesheets/common/topic-timeline.scss

@@ -253,6 +253,10 @@
       cursor: ns-resize;
       display: flex;
       align-items: center;
+
+      .back-button {
+        margin-top: 1em;
+      }
     }
 
     .timeline-replies {
@@ -263,10 +267,6 @@
       position: absolute;
       margin-left: -0.35em;
 
-      .btn-small {
-        padding: 2px 5px;
-      }
-
       i.progress {
         font-size: 0.8em;
         color: $tertiary;
@@ -274,6 +274,10 @@
       }
     }
 
+    .back-button {
+      padding: 2px 5px;
+    }
+
     .now-date {
       @include unselectable;
       display: inline-block;