New: Add html class when page scroll is at the top (fix #675)

js/app.js

@@ -17,6 +17,7 @@ import 'core/js/models/lockingModel';
 import 'core/js/mpabc';
 import 'core/js/helpers';
 import 'core/js/scrolling';
+import 'core/js/scrollPositionTracker';
 import 'core/js/headings';
 import 'core/js/navigation';
 import 'core/js/startController';

js/scrollPositionTracker.js

@@ -0,0 +1,51 @@
+import Adapt from 'core/js/adapt';
+import scrolling from 'core/js/scrolling';
+
+class ScrollPositionTracker extends Backbone.Controller {
+
+  initialize() {
+    this.$html = $('html');
+    this._scrollContainer = null;
+    this._isCheckingScroll = false;
+    this.checkScrollPosition = this.checkScrollPosition.bind(this);
+    this.onScroll = this.onScroll.bind(this);
+
+    this.listenToOnce(Adapt, 'configModel:dataLoaded', this.setup);
+  }
+
+  setup() {
+    this._scrollContainer = scrolling.isLegacyScrolling ? window : document.body;
+
+    this.checkScrollPosition();
+
+    this.listenTo(Adapt, {
+      'contentObjectView:ready': this.onContentObjectReady,
+      'contentObjectView:preRemove': this.onContentObjectPreRemove
+    });
+  }
+
+  onContentObjectReady() {
+    $(this._scrollContainer).on('scroll.scrollPositionTracker', this.onScroll);
+    this.checkScrollPosition();
+  }
+
+  onContentObjectPreRemove() {
+    $(this._scrollContainer).off('scroll.scrollPositionTracker');
+  }
+
+  onScroll() {
+    if (this._isCheckingScroll) return;
+    this._isCheckingScroll = true;
+    requestAnimationFrame(this.checkScrollPosition);
+  }
+
+  checkScrollPosition() {
+    this._isCheckingScroll = false;
+    const scrollY = window.scrollY || window.pageYOffset || 0;
+    const isAtTop = (scrollY === 0);
+    this.$html.toggleClass('is-scroll-at-top', isAtTop);
+  }
+
+}
+
+export default new ScrollPositionTracker();