Optimize ScrollListener performance

Listen to "scroll" event and throttle callback executions instead
of actively polling for changes to the scroll position.

Fixes #1222.

framework/core/js/src/common/utils/ScrollListener.js

@@ -1,4 +1,4 @@
-const scroll = window.requestAnimationFrame ||
+const later = window.requestAnimationFrame ||
   window.webkitRequestAnimationFrame ||
   window.mozRequestAnimationFrame ||
   window.msRequestAnimationFrame ||
@@ -17,7 +17,7 @@ export default class ScrollListener {
    */
   constructor(callback) {
     this.callback = callback;
-    this.lastTop = -1;
+    this.ticking = false;
   }
 
   /**
@@ -27,27 +27,27 @@ export default class ScrollListener {
    * @protected
    */
   loop() {
-    if (!this.active) return;
+    // THROTTLE: If the callback is still running (or hasn't yet run), we ignore
+    // further scroll events.
+    if (this.ticking) return;
 
+    // Schedule the callback to be executed soon (TM), and stop throttling once
+    // the callback is done.
+    later(() => {
       this.update();
+      this.ticking = false;
+    });
 
-    scroll(this.loop.bind(this));
+    this.ticking = true;
   }
 
   /**
-   * Check if the scroll position has changed; if it has, run the handler.
+   * Run the callback, whether there was a scroll event or not.
    *
-   * @param {Boolean} [force=false] Whether or not to force the handler to be
-   *     run, even if the scroll position hasn't changed.
    * @public
    */
-  update(force) {
+  update() {
-    const top = window.pageYOffset;
+    this.callback(window.pageYOffset);
-
-    if (this.lastTop !== top || force) {
-      this.callback(top);
-      this.lastTop = top;
-    }
   }
 
   /**
@@ -57,8 +57,10 @@ export default class ScrollListener {
    */
   start() {
     if (!this.active) {
-      this.active = true;
+      window.addEventListener(
-      this.loop();
+        'scroll',
+        this.active = this.loop.bind(this)
+      );
     }
   }
 
@@ -68,6 +70,8 @@ export default class ScrollListener {
    * @public
    */
   stop() {
-    this.active = false;
+    window.removeEventListener('scroll', this.active);
+
+    this.active = null;
   }
 }

framework/core/js/src/forum/components/PostStream.js

@@ -2,7 +2,6 @@ import Component from '../../common/Component';
 import ScrollListener from '../../common/utils/ScrollListener';
 import PostLoading from './LoadingPost';
 import anchorScroll from '../../common/utils/anchorScroll';
-import mixin from '../../common/utils/mixin';
 import evented from '../../common/utils/evented';
 import ReplyPlaceholder from './ReplyPlaceholder';
 import Button from '../../common/components/Button';
@@ -586,7 +585,7 @@ class PostStream extends Component {
    */
   unpause() {
     this.paused = false;
-    this.scrollListener.update(true);
+    this.scrollListener.update();
     this.trigger('unpaused');
   }
 }

framework/core/js/src/forum/components/PostStreamScrubber.js

@@ -2,7 +2,6 @@ import Component from '../../common/Component';
 import icon from '../../common/helpers/icon';
 import ScrollListener from '../../common/utils/ScrollListener';
 import SubtreeRetainer from '../../common/utils/SubtreeRetainer';
-import computed from '../../common/utils/computed';
 import formatNumber from '../../common/utils/formatNumber';
 
 /**
@@ -365,7 +364,7 @@ export default class PostStreamScrubber extends Component {
   }
 
   onresize() {
-    this.scrollListener.update(true);
+    this.scrollListener.update();
 
     // Adjust the height of the scrollbar so that it fills the height of
     // the sidebar and doesn't overlap the footer.