Revert 4a48d03b31 (fixes #348)

Since 4a48d03b31, IntersectionObserverArticle
assumes that its children do not change unless the number of children changes.

This is not the case with the notification overlay, which resulted in the
checkmark of notification cleaning mode not updating unless scrolling to make
notifications appear/disappear.

This change may negatively impact performances.

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

@@ -1,15 +1,10 @@
 import React from 'react';
 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';
 
-// Diff these props in the "rendered" state
+export default class IntersectionObserverArticle extends ImmutablePureComponent {
-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,
@@ -27,15 +22,18 @@ export default class IntersectionObserverArticle extends React.Component {
   }
 
   shouldComponentUpdate (nextProps, nextState) {
-    const isUnrendered = !this.state.isIntersecting && (this.state.isHidden || this.props.cachedHeight);
+    if (!nextState.isIntersecting && nextState.isHidden) {
-    const willBeUnrendered = !nextState.isIntersecting && (nextState.isHidden || nextProps.cachedHeight);
+      // It's only if we're not intersecting (i.e. offscreen) and isHidden is true
-    if (!!isUnrendered !== !!willBeUnrendered) {
+      // that either "isIntersecting" or "isHidden" matter, and then they're
-      // If we're going from rendered to unrendered (or vice versa) then update
+      // 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
       return true;
     }
-    // Otherwise, diff based on props
+    // Otherwise, diff based on "updateOnProps" and "updateOnStates"
-    const propsToDiff = isUnrendered ? updateOnPropsForUnrendered : updateOnPropsForRendered;
+    return super.shouldComponentUpdate(nextProps, nextState);
-    return !propsToDiff.every(prop => is(nextProps[prop], this.props[prop]));
   }
 
   componentDidMount () {