Revert to using upstream's optimisations

This *does* break things, as `shouldComponentUpdate` assume the
children to never change!

app/javascript/flavours/glitch/components/intersection_observer_article.js

@@ -3,8 +3,14 @@ import PropTypes from 'prop-types';
 import ImmutablePureComponent from 'react-immutable-pure-component';
 import scheduleIdleTask from 'flavours/glitch/util/schedule_idle_task';
 import getRectFromEntry from 'flavours/glitch/util/get_rect_from_entry';
+import { is } from 'immutable';
 
-export default class IntersectionObserverArticle extends ImmutablePureComponent {
+// Diff these props in the "rendered" state
+const updateOnPropsForRendered = ['id', 'index', 'listLength'];
+// Diff these props in the "unrendered" state
+const updateOnPropsForUnrendered = ['id', 'index', 'listLength', 'cachedHeight'];
+
+export default class IntersectionObserverArticle extends React.Component {
 
   static propTypes = {
     intersectionObserverWrapper: PropTypes.object.isRequired,
@@ -22,20 +28,18 @@ export default class IntersectionObserverArticle extends ImmutablePureComponent
   }
 
   shouldComponentUpdate (nextProps, nextState) {
-    if (!nextState.isIntersecting && nextState.isHidden) {
-      // It's only if we're not intersecting (i.e. offscreen) and isHidden is true
-      // that either "isIntersecting" or "isHidden" matter, and then they're
-      // the only things that matter (and updated ARIA attributes).
-      return this.state.isIntersecting || !this.state.isHidden || nextProps.listLength !== this.props.listLength;
-    } else if (nextState.isIntersecting && !this.state.isIntersecting) {
-      // If we're going from a non-intersecting state to an intersecting state,
-      // (i.e. offscreen to onscreen), then we definitely need to re-render
+    const isUnrendered = !this.state.isIntersecting && (this.state.isHidden || this.props.cachedHeight);
+    const willBeUnrendered = !nextState.isIntersecting && (nextState.isHidden || nextProps.cachedHeight);
+    if (!!isUnrendered !== !!willBeUnrendered) {
+      // If we're going from rendered to unrendered (or vice versa) then update
       return true;
     }
-    // Otherwise, diff based on "updateOnProps" and "updateOnStates"
-    return super.shouldComponentUpdate(nextProps, nextState);
+    // Otherwise, diff based on props
+    const propsToDiff = isUnrendered ? updateOnPropsForUnrendered : updateOnPropsForRendered;
+    return !propsToDiff.every(prop => is(nextProps[prop], this.props[prop]));
   }
 
+
   componentDidMount () {
     const { intersectionObserverWrapper, id } = this.props;