Summary: fix slowness due to layout thrashing when reloading a large … (#12661)

* Summary: fix slowness due to layout thrashing when reloading a large set of status updates

in order to limit the maximum size of a status in a list view (e.g. the home timeline), so as to avoid having to scroll all the way through an abnormally large status update (see https://github.com/tootsuite/mastodon/pull/8205), the following steps are taken:
•the element containing the status is rendered in the browser
•its height is calculated, to determine if it exceeds the maximum height threshold.
Unfortunately for performance, these steps are carried out in the componentDidMount(/Update) method, which also performs style modifications on the element.  The combination of  height request and style modification during javascript evaluation in the browser leads to layout-thrashing, where the elements are repeatedly re-laid-out (see https://developers.google.com/web/fundamentals/performance/rendering/avoid-large-complex-layouts-and-layout-thrashing & https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Performance_best_practices_for_Firefox_fe_engineers).
The solution implemented here is to memoize the collapsed state in Redux the first time the status is seen (e.g. when fetched as part of a small batch, to populate the home timeline) , so that on subsequent re-renders, the value can be queried, rather than recalculated.  This strategy is derived from https://github.com/tootsuite/mastodon/pull/4439 & https://github.com/tootsuite/mastodon/pull/4909, and should resolve https://github.com/tootsuite/mastodon/issues/12455.

Andrew Lin (https://github.com/onethreeseven) is thanked for his assistance in root cause analysis and solution brainstorming

* remove getSnapshotBeforeUpdate from status

* remove componentWillUnmount from status

* persist last-intersected status update and restore when ScrollableList is restored

e.g. when navigating from home-timeline to a status conversational  thread and <Back again

* cache currently-viewing status id to avoid calling redux with identical value

* refactor collapse toggle to pass explicit boolean

11 files changed, 74 insertions, 35 deletions

diff --git a/app/javascript/mastodon/actions/statuses.js b/app/javascript/mastodon/actions/statuses.js
index 06a19afc3..5640201c6 100644
--- a/app/javascript/mastodon/actions/statuses.js
+++ b/app/javascript/mastodon/actions/statuses.js
@@ -26,8 +26,9 @@ export const STATUS_UNMUTE_REQUEST = 'STATUS_UNMUTE_REQUEST';
 export const STATUS_UNMUTE_SUCCESS = 'STATUS_UNMUTE_SUCCESS';
 export const STATUS_UNMUTE_FAIL    = 'STATUS_UNMUTE_FAIL';
 
-export const STATUS_REVEAL = 'STATUS_REVEAL';
-export const STATUS_HIDE   = 'STATUS_HIDE';
+export const STATUS_REVEAL   = 'STATUS_REVEAL';
+export const STATUS_HIDE     = 'STATUS_HIDE';
+export const STATUS_COLLAPSE = 'STATUS_COLLAPSE';
 
 export const REDRAFT = 'REDRAFT';
 
@@ -320,3 +321,11 @@ export function revealStatus(ids) {
     ids,
   };
 };
+
+export function toggleStatusCollapse(id, isCollapsed) {
+  return {
+    type: STATUS_COLLAPSE,
+    id,
+    isCollapsed,
+  };
+}
diff --git a/app/javascript/mastodon/actions/timelines.js b/app/javascript/mastodon/actions/timelines.js
index bc2ac5e82..1a634b55d 100644
--- a/app/javascript/mastodon/actions/timelines.js
+++ b/app/javascript/mastodon/actions/timelines.js
@@ -17,6 +17,14 @@ export const TIMELINE_LOAD_PENDING = 'TIMELINE_LOAD_PENDING';
 export const TIMELINE_DISCONNECT   = 'TIMELINE_DISCONNECT';
 export const TIMELINE_CONNECT      = 'TIMELINE_CONNECT';
 
+export const CURRENTLY_VIEWING = 'CURRENTLY_VIEWING';
+
+export const updateCurrentlyViewing = (timeline, id) => ({
+  type: CURRENTLY_VIEWING,
+  timeline,
+  id,
+});
+
 export const loadPending = timeline => ({
   type: TIMELINE_LOAD_PENDING,
   timeline,
diff --git a/app/javascript/mastodon/components/intersection_observer_article.js b/app/javascript/mastodon/components/intersection_observer_article.js
index e453730ba..d475e5d1c 100644
--- a/app/javascript/mastodon/components/intersection_observer_article.js
+++ b/app/javascript/mastodon/components/intersection_observer_article.js
@@ -20,6 +20,8 @@ export default class IntersectionObserverArticle extends React.Component {
     cachedHeight: PropTypes.number,
     onHeightChange: PropTypes.func,
     children: PropTypes.node,
+    currentlyViewing: PropTypes.number,
+    updateCurrentlyViewing: PropTypes.func,
   };
 
   state = {
@@ -48,6 +50,8 @@ export default class IntersectionObserverArticle extends React.Component {
     );
 
     this.componentMounted = true;
+
+    if(id === this.props.currentlyViewing) this.node.scrollIntoView();
   }
 
   componentWillUnmount () {
@@ -60,6 +64,8 @@ export default class IntersectionObserverArticle extends React.Component {
   handleIntersection = (entry) => {
     this.entry = entry;
 
+    if(entry.intersectionRatio > 0.75 && this.props.updateCurrentlyViewing) this.props.updateCurrentlyViewing(this.id);
+
     scheduleIdleTask(this.calculateHeight);
     this.setState(this.updateStateAfterIntersection);
   }
diff --git a/app/javascript/mastodon/components/scrollable_list.js b/app/javascript/mastodon/components/scrollable_list.js
index 421756803..6338ccd5c 100644
--- a/app/javascript/mastodon/components/scrollable_list.js
+++ b/app/javascript/mastodon/components/scrollable_list.js
@@ -36,6 +36,8 @@ export default class ScrollableList extends PureComponent {
     emptyMessage: PropTypes.node,
     children: PropTypes.node,
     bindToDocument: PropTypes.bool,
+    currentlyViewing: PropTypes.number,
+    updateCurrentlyViewing: PropTypes.func,
   };
 
   static defaultProps = {
@@ -309,6 +311,8 @@ export default class ScrollableList extends PureComponent {
                 listLength={childrenCount}
                 intersectionObserverWrapper={this.intersectionObserverWrapper}
                 saveHeightKey={trackScroll ? `${this.context.router.route.location.key}:${scrollKey}` : null}
+                currentlyViewing={this.props.currentlyViewing}
+                updateCurrentlyViewing={this.props.updateCurrentlyViewing}
               >
                 {React.cloneElement(child, {
                   getScrollPosition: this.getScrollPosition,
diff --git a/app/javascript/mastodon/components/status.js b/app/javascript/mastodon/components/status.js
index e120278a0..12fc4a9a6 100644
--- a/app/javascript/mastodon/components/status.js
+++ b/app/javascript/mastodon/components/status.js
@@ -76,6 +76,7 @@ class Status extends ImmutablePureComponent {
     onEmbed: PropTypes.func,
     onHeightChange: PropTypes.func,
     onToggleHidden: PropTypes.func,
+    onToggleCollapsed: PropTypes.func,
     muted: PropTypes.bool,
     hidden: PropTypes.bool,
     unread: PropTypes.bool,
@@ -107,14 +108,6 @@ class Status extends ImmutablePureComponent {
     this.didShowCard = !this.props.muted && !this.props.hidden && this.props.status && this.props.status.get('card');
   }
 
-  getSnapshotBeforeUpdate () {
-    if (this.props.getScrollPosition) {
-      return this.props.getScrollPosition();
-    } else {
-      return null;
-    }
-  }
-
   static getDerivedStateFromProps(nextProps, prevState) {
     if (nextProps.status && nextProps.status.get('id') !== prevState.statusId) {
       return {
@@ -141,17 +134,6 @@ class Status extends ImmutablePureComponent {
     }
   }
 
-  componentWillUnmount() {
-    if (this.node && this.props.getScrollPosition) {
-      const position = this.props.getScrollPosition();
-      if (position !== null && this.node.offsetTop < position.top) {
-        requestAnimationFrame(() => {
-          this.props.updateScrollBottom(position.height - position.top);
-        });
-      }
-    }
-  }
-
   handleToggleMediaVisibility = () => {
     this.setState({ showMedia: !this.state.showMedia });
   }
@@ -196,7 +178,11 @@ class Status extends ImmutablePureComponent {
 
   handleExpandedToggle = () => {
     this.props.onToggleHidden(this._properStatus());
-  };
+  }
+
+  handleCollapsedToggle = isCollapsed => {
+    this.props.onToggleCollapsed(this._properStatus(), isCollapsed);
+  }
 
   renderLoadingMediaGallery () {
     return <div className='media-gallery' style={{ height: '110px' }} />;
@@ -466,7 +452,7 @@ class Status extends ImmutablePureComponent {
               </a>
             </div>
 
-            <StatusContent status={status} onClick={this.handleClick} expanded={!status.get('hidden')} onExpandedToggle={this.handleExpandedToggle} collapsable />
+            <StatusContent status={status} onClick={this.handleClick} expanded={!status.get('hidden')} onExpandedToggle={this.handleExpandedToggle} collapsable onCollapsedToggle={this.handleCollapsedToggle} />
 
             {media}
 
diff --git a/app/javascript/mastodon/components/status_content.js b/app/javascript/mastodon/components/status_content.js
index d13091325..5d921fd41 100644
--- a/app/javascript/mastodon/components/status_content.js
+++ b/app/javascript/mastodon/components/status_content.js
@@ -23,11 +23,11 @@ export default class StatusContent extends React.PureComponent {
     onExpandedToggle: PropTypes.func,
     onClick: PropTypes.func,
     collapsable: PropTypes.bool,
+    onCollapsedToggle: PropTypes.func,
   };
 
   state = {
     hidden: true,
-    collapsed: null, //  `collapsed: null` indicates that an element doesn't need collapsing, while `true` or `false` indicates that it does (and is/isn't).
   };
 
   _updateStatusLinks () {
@@ -62,14 +62,16 @@ export default class StatusContent extends React.PureComponent {
       link.setAttribute('rel', 'noopener noreferrer');
     }
 
-    if (
-      this.props.collapsable
-      && this.props.onClick
-      && this.state.collapsed === null
-      && node.clientHeight > MAX_HEIGHT
-      && this.props.status.get('spoiler_text').length === 0
-    ) {
-      this.setState({ collapsed: true });
+    if (this.props.status.get('collapsed', null) === null) {
+      let collapsed =
+          this.props.collapsable
+          && this.props.onClick
+          && node.clientHeight > MAX_HEIGHT
+          && this.props.status.get('spoiler_text').length === 0;
+
+      if(this.props.onCollapsedToggle) this.props.onCollapsedToggle(collapsed);
+
+      this.props.status.set('collapsed', collapsed);
     }
   }
 
@@ -178,6 +180,7 @@ export default class StatusContent extends React.PureComponent {
     }
 
     const hidden = this.props.onExpandedToggle ? !this.props.expanded : this.state.hidden;
+    const renderReadMore = this.props.onClick && status.get('collapsed');
 
     const content = { __html: status.get('contentHtml') };
     const spoilerContent = { __html: status.get('spoilerHtml') };
@@ -185,7 +188,7 @@ export default class StatusContent extends React.PureComponent {
     const classNames = classnames('status__content', {
       'status__content--with-action': this.props.onClick && this.context.router,
       'status__content--with-spoiler': status.get('spoiler_text').length > 0,
-      'status__content--collapsed': this.state.collapsed === true,
+      'status__content--collapsed': renderReadMore,
     });
 
     if (isRtl(status.get('search_index'))) {
@@ -237,7 +240,7 @@ export default class StatusContent extends React.PureComponent {
         </div>,
       ];
 
-      if (this.state.collapsed) {
+      if (renderReadMore) {
         output.push(readMoreButton);
       }
 
diff --git a/app/javascript/mastodon/components/status_list.js b/app/javascript/mastodon/components/status_list.js
index e1b370c91..82e069601 100644
--- a/app/javascript/mastodon/components/status_list.js
+++ b/app/javascript/mastodon/components/status_list.js
@@ -26,6 +26,8 @@ export default class StatusList extends ImmutablePureComponent {
     emptyMessage: PropTypes.node,
     alwaysPrepend: PropTypes.bool,
     timelineId: PropTypes.string,
+    currentlyViewing: PropTypes.number,
+    updateCurrentlyViewing: PropTypes.func,
   };
 
   static defaultProps = {
@@ -58,6 +60,12 @@ export default class StatusList extends ImmutablePureComponent {
     this.props.onLoadMore(this.props.statusIds.size > 0 ? this.props.statusIds.last() : undefined);
   }, 300, { leading: true })
 
+  updateCurrentlyViewingWithCache = (id) => {
+    if(this.cachedCurrentlyViewing === id) return;
+    this.cachedCurrentlyViewing = id;
+    this.props.updateCurrentlyViewing(id);
+  }
+
   _selectChild (index, align_top) {
     const container = this.node.node;
     const element = container.querySelector(`article:nth-of-type(${index + 1}) .focusable`);
@@ -79,6 +87,7 @@ export default class StatusList extends ImmutablePureComponent {
   render () {
     const { statusIds, featuredStatusIds, shouldUpdateScroll, onLoadMore, timelineId, ...other }  = this.props;
     const { isLoading, isPartial } = other;
+    other.updateCurrentlyViewing = this.updateCurrentlyViewingWithCache;
 
     if (isPartial) {
       return <RegenerationIndicator />;
diff --git a/app/javascript/mastodon/containers/status_container.js b/app/javascript/mastodon/containers/status_container.js
index 35c16a20c..2ba3a3123 100644
--- a/app/javascript/mastodon/containers/status_container.js
+++ b/app/javascript/mastodon/containers/status_container.js
@@ -23,6 +23,7 @@ import {
   deleteStatus,
   hideStatus,
   revealStatus,
+  toggleStatusCollapse,
 } from '../actions/statuses';
 import {
   unmuteAccount,
@@ -190,6 +191,10 @@ const mapDispatchToProps = (dispatch, { intl }) => ({
     }
   },
 
+  onToggleCollapsed (status, isCollapsed) {
+    dispatch(toggleStatusCollapse(status.get('id'), isCollapsed));
+  },
+
   onBlockDomain (domain) {
     dispatch(openModal('CONFIRM', {
       message: <FormattedMessage id='confirmations.domain_block.message' defaultMessage='Are you really, really sure you want to block the entire {domain}? In most cases a few targeted blocks or mutes are sufficient and preferable. You will not see content from that domain in any public timelines or your notifications. Your followers from that domain will be removed.' values={{ domain: <strong>{domain}</strong> }} />,
diff --git a/app/javascript/mastodon/features/ui/containers/status_list_container.js b/app/javascript/mastodon/features/ui/containers/status_list_container.js
index 9f6cbf988..33af628ca 100644
--- a/app/javascript/mastodon/features/ui/containers/status_list_container.js
+++ b/app/javascript/mastodon/features/ui/containers/status_list_container.js
@@ -1,6 +1,6 @@
 import { connect } from 'react-redux';
 import StatusList from '../../../components/status_list';
-import { scrollTopTimeline, loadPending } from '../../../actions/timelines';
+import { scrollTopTimeline, loadPending, updateCurrentlyViewing } from '../../../actions/timelines';
 import { Map as ImmutableMap, List as ImmutableList } from 'immutable';
 import { createSelector } from 'reselect';
 import { debounce } from 'lodash';
@@ -39,6 +39,7 @@ const makeMapStateToProps = () => {
     isPartial: state.getIn(['timelines', timelineId, 'isPartial'], false),
     hasMore:   state.getIn(['timelines', timelineId, 'hasMore']),
     numPending: getPendingStatusIds(state, { type: timelineId }).size,
+    currentlyViewing: state.getIn(['timelines', timelineId, 'currentlyViewing'], -1),
   });
 
   return mapStateToProps;
@@ -56,6 +57,7 @@ const mapDispatchToProps = (dispatch, { timelineId }) => ({
 
   onLoadPending: () => dispatch(loadPending(timelineId)),
 
+  updateCurrentlyViewing: id => dispatch(updateCurrentlyViewing(timelineId, id)),
 });
 
 export default connect(makeMapStateToProps, mapDispatchToProps)(StatusList);
diff --git a/app/javascript/mastodon/reducers/statuses.js b/app/javascript/mastodon/reducers/statuses.js
index 772f98bcb..398a48cff 100644
--- a/app/javascript/mastodon/reducers/statuses.js
+++ b/app/javascript/mastodon/reducers/statuses.js
@@ -12,6 +12,7 @@ import {
   STATUS_UNMUTE_SUCCESS,
   STATUS_REVEAL,
   STATUS_HIDE,
+  STATUS_COLLAPSE,
 } from '../actions/statuses';
 import { TIMELINE_DELETE } from '../actions/timelines';
 import { STATUS_IMPORT, STATUSES_IMPORT } from '../actions/importer';
@@ -73,6 +74,8 @@ export default function statuses(state = initialState, action) {
         }
       });
     });
+  case STATUS_COLLAPSE:
+    return state.setIn([action.id, 'collapsed'], action.isCollapsed);
   case TIMELINE_DELETE:
     return deleteStatus(state, action.id, action.references);
   default:
diff --git a/app/javascript/mastodon/reducers/timelines.js b/app/javascript/mastodon/reducers/timelines.js
index 0d7222e10..970db425e 100644
--- a/app/javascript/mastodon/reducers/timelines.js
+++ b/app/javascript/mastodon/reducers/timelines.js
@@ -9,6 +9,7 @@ import {
   TIMELINE_CONNECT,
   TIMELINE_DISCONNECT,
   TIMELINE_LOAD_PENDING,
+  CURRENTLY_VIEWING,
 } from '../actions/timelines';
 import {
   ACCOUNT_BLOCK_SUCCESS,
@@ -28,6 +29,7 @@ const initialTimeline = ImmutableMap({
   hasMore: true,
   pendingItems: ImmutableList(),
   items: ImmutableList(),
+  currentlyViewing: -1,
 });
 
 const expandNormalizedTimeline = (state, timeline, statuses, next, isPartial, isLoadingRecent, usePendingItems) => {
@@ -168,6 +170,8 @@ export default function timelines(state = initialState, action) {
       initialTimeline,
       map => map.set('online', false).update(action.usePendingItems ? 'pendingItems' : 'items', items => items.first() ? items.unshift(null) : items)
     );
+  case CURRENTLY_VIEWING:
+    return state.update(action.timeline, initialTimeline, map => map.set('currentlyViewing', action.id));
   default:
     return state;
   }