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

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 +321,10 @@ export function revealStatus(ids) {
     ids,
   };
 };
+
+export function toggleStatusCollapse(id) {
+  return {
+    type: STATUS_COLLAPSE,
+    id,
+  };
+}

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,
@@ -196,7 +197,11 @@ class Status extends ImmutablePureComponent {
 
   handleExpandedToggle = () => {
     this.props.onToggleHidden(this._properStatus());
-  };
+  }
+
+  handleCollapsedToggle = () => {
+    this.props.onToggleCollapsed(this._properStatus());
+  }
 
   renderLoadingMediaGallery () {
     return <div className='media-gallery' style={{ height: '110px' }} />;
@@ -466,7 +471,7 @@ 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}
 

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 +62,24 @@ 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) {
+        if(collapsed) {
+          this.props.onCollapsedToggle();
+        } else {
+          //toggle twice, to go from null→true→false
+          this.props.onCollapsedToggle();
+          this.props.onCollapsedToggle();
+        }
+      }
+
+      this.props.status.set('collapsed', collapsed);
     }
   }
 
@@ -178,6 +188,7 @@ 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 +196,7 @@ 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 +248,7 @@ export default class StatusContent extends React.PureComponent {
         </div>,
       ];
 
-      if (this.state.collapsed) {
+      if (renderReadMore) {
         output.push(readMoreButton);
       }
 

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) {
+    dispatch(toggleStatusCollapse(status.get('id')));
+  },
+
   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> }} />,

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.updateIn([action.id, 'collapsed'], old => !old);
   case TIMELINE_DELETE:
     return deleteStatus(state, action.id, action.references);
   default: