perf(timeline): memoize message rows and cache the per-render call-aggregate scan and HTML body parse

src/app/components/message/RenderBody.tsx

@@ -1,4 +1,4 @@
-import React from 'react';
+import React, { useMemo } from 'react';
 import parse, { HTMLReactParserOptions } from 'html-react-parser';
 import Linkify from 'linkify-react';
 import { Opts } from 'linkifyjs';
@@ -21,16 +21,30 @@ export function RenderBody({
   htmlReactParserOptions,
   linkifyOpts,
 }: RenderBodyProps) {
+  // `sanitizeCustomHtml` (a full sanitize-html DOM pass) + `parse` are the
+  // hottest per-render cost on the timeline — RoomTimeline re-renders every
+  // visible Message on each live event. Both `customBody` and the memoized
+  // `htmlReactParserOptions` (RoomTimeline.tsx:654) are stable across those
+  // renders, so caching the parsed tree skips the work for unchanged rows.
+  const parsedCustomBody = useMemo(
+    () => (customBody ? parse(sanitizeCustomHtml(customBody), htmlReactParserOptions) : null),
+    [customBody, htmlReactParserOptions]
+  );
+  // Plaintext path: scaleSystemEmoji walks the string for unicode emoji and
+  // highlightText re-splits on the search regex — both pure functions of
+  // (body, highlightRegex), so memoize to match the custom-body fast path.
+  const plainBody = useMemo(
+    () =>
+      highlightRegex
+        ? highlightText(highlightRegex, scaleSystemEmoji(body))
+        : scaleSystemEmoji(body),
+    [body, highlightRegex]
+  );
+
   if (body === '') <MessageEmptyContent />;
   if (customBody) {
     if (customBody === '') <MessageEmptyContent />;
-    return parse(sanitizeCustomHtml(customBody), htmlReactParserOptions);
+    return parsedCustomBody;
   }
-  return (
+  return <Linkify options={linkifyOpts}>{plainBody}</Linkify>;
-    <Linkify options={linkifyOpts}>
-      {highlightRegex
-        ? highlightText(highlightRegex, scaleSystemEmoji(body))
-        : scaleSystemEmoji(body)}
-    </Linkify>
-  );
 }

src/app/features/room/RoomTimeline.tsx

@@ -83,6 +83,7 @@ import {
   SyslineMessage,
   EncryptedContent,
   useMessageInteractionHandlers,
+  getMessageRenderSig,
 } from './message';
 import { ThreadSummaryCard } from './ThreadSummaryCard';
 import { useMemberEventParser } from '../../hooks/useMemberEventParser';
@@ -1274,7 +1275,13 @@ export function RoomTimeline({
   // created_ts + expires, we treat the stale join as gone. Linear in
   // #call-member events (rare); matches the inline `streamRenderableItemHasBefore`
   // IIFE pattern just below.
-  const callAnchors: Map<string, CallAggregate> = (() => {
+  // This rebuilds the call-membership state machine by scanning EVERY event in
+  // EVERY linked timeline (grows unbounded with scrollback). Previously it ran
+  // as a per-render IIFE, so every fab/atBottom/focus/unread/pulse state change
+  // re-scanned the whole room. Memoized on the `timeline` state object — the
+  // only reactive input — so it recomputes only when timeline content changes.
+  // Date.now() (used below for «ongoing») reads fresh on each actual recompute.
+  const callAnchors: Map<string, CallAggregate> = useMemo(() => {
     type CallScan = {
       slotId: string;
       anchorEventId: string;
@@ -1489,7 +1496,7 @@ export function RoomTimeline({
     }
     /* eslint-enable no-restricted-syntax, no-continue */
     return anchors;
-  })();
+  }, [timeline]);
 
   const renderMatrixEvent = useMatrixEventRenderer<
     [string, MatrixEvent, number, EventTimelineSet, boolean, boolean, boolean]
@@ -1528,6 +1535,7 @@ export function RoomTimeline({
         return (
           <Message
             key={mEvent.getId()}
+            renderSig={getMessageRenderSig(room, mEvent, editedEvent, showUrlPreview)}
             data-message-item={item}
             data-message-id={mEventId}
             room={room}
@@ -1637,6 +1645,12 @@ export function RoomTimeline({
               return (
                 <Message
                   key={mEvent.getId()}
+                  renderSig={getMessageRenderSig(
+                    room,
+                    mEvent,
+                    getEditedEvent(mEventId, mEvent, timelineSet),
+                    showUrlPreview
+                  )}
                   data-message-item={item}
                   data-message-id={mEventId}
                   room={room}
@@ -1776,6 +1790,7 @@ export function RoomTimeline({
         return (
           <Message
             key={mEvent.getId()}
+            renderSig={getMessageRenderSig(room, mEvent, undefined, showUrlPreview)}
             data-message-item={item}
             data-message-id={mEventId}
             room={room}

src/app/features/room/ThreadDrawer.tsx

@@ -47,7 +47,13 @@ import { ImageViewer } from '../../components/image-viewer';
 import { RenderMessageContent } from '../../components/RenderMessageContent';
 import { RoomInput } from './RoomInput';
 import { RoomInputPlaceholder } from './RoomInputPlaceholder';
-import { EncryptedContent, Message, Reactions, useMessageInteractionHandlers } from './message';
+import {
+  EncryptedContent,
+  Message,
+  Reactions,
+  getMessageRenderSig,
+  useMessageInteractionHandlers,
+} from './message';
 import { useRoomPermissions } from '../../hooks/useRoomPermissions';
 import { useRoomCreators } from '../../hooks/useRoomCreators';
 import { usePowerLevelsContext } from '../../hooks/usePowerLevels';
@@ -998,6 +1004,12 @@ export function ThreadDrawer({ room, rootId, parentRoomPath, variant }: ThreadDr
       return (
         <Message
           key={eventId}
+          renderSig={getMessageRenderSig(
+            room,
+            mEvent,
+            getEditedEvent(eventId, mEvent, timelineSet),
+            showUrlPreview
+          )}
           data-message-id={eventId}
           room={room}
           mEvent={mEvent}

src/app/features/room/message/Message.tsx

@@ -23,7 +23,9 @@ import {
   config,
 } from 'folds';
 import React, {
+  ComponentProps,
   FormEventHandler,
+  memo,
   MouseEventHandler,
   ReactNode,
   useCallback,
@@ -713,8 +715,12 @@ export type MessageProps = {
   // caller passes `true` alongside `layout='channel'`; kept as a prop so
   // un-bubbled `ChannelLayout` consumers stay possible.
   channelHeaderInBubble?: boolean;
+  // Parent-computed render snapshot (see getMessageRenderSig). Drives the
+  // React.memo comparator so in-place MatrixEvent mutations (edit/redact/
+  // decrypt/echo) and rename/urlPreview changes repaint the row.
+  renderSig?: string;
 };
-export const Message = as<'div', MessageProps>(
+const MessageInner = as<'div', MessageProps>(
   (
     {
       className,
@@ -1246,6 +1252,107 @@ export const Message = as<'div', MessageProps>(
   }
 );
 
+// Parent-computed render snapshot for a timeline message row. This MUST be
+// called during the PARENT's render and threaded in as the `renderSig` prop;
+// it must NOT be recomputed from `mEvent` inside the memo comparator.
+//
+// matrix-js-sdk mutates the `MatrixEvent` IN PLACE on edit / redaction /
+// decryption / local-echo (makeReplaced / makeRedacted / setClearData /
+// handleRemoteEcho), and the timeline reuses one MatrixEvent instance per id,
+// so `prevProps.mEvent === nextProps.mEvent`. A signature read off that live
+// object inside the comparator would return the SAME (already-mutated) value
+// for both prev and next and could never detect the change -- the classic
+// stale-row trap element-web documents on `EventTile` (`isRedacted` is a prop
+// there for exactly this reason). Capturing the snapshot here freezes it into
+// the prop value, so React's retained prevProps holds the OLD snapshot and the
+// comparator can diff old-vs-new. It also folds in the sender display name and
+// the urlPreview setting, so a live rename / preview-toggle repaints the row.
+export function getMessageRenderSig(
+  room: Room,
+  mEvent: MatrixEvent,
+  editedEvent: MatrixEvent | undefined,
+  urlPreview: boolean | undefined
+): string {
+  const senderId = mEvent.getSender() ?? '';
+  const senderDisplayName =
+    getMemberDisplayName(room, senderId) ?? getMxIdLocalPart(senderId) ?? senderId;
+  const content = mEvent.getContent();
+  return [
+    mEvent.getId() ?? '',
+    mEvent.getType(),
+    typeof content.msgtype === 'string' ? content.msgtype : '',
+    mEvent.isRedacted() ? '1' : '0',
+    editedEvent?.getId() ?? '',
+    mEvent.status ?? '',
+    senderDisplayName,
+    urlPreview ? '1' : '0',
+  ].join('\u0001');
+}
+
+// Custom equality for the timeline row. RoomTimeline re-renders on every live
+// event / receipt / scroll-state change and rebuilds all ~80 visible rows; this
+// lets React skip rows whose visible output is unchanged. SAFE because the
+// dynamic children self-subscribe to their own data (Reactions→useRelations,
+// Reply→useRoomEvent, ThreadSummaryCard→ThreadEvent) and the delivery dot
+// self-subscribes (useDotColor→RoomEvent.Receipt) — so we only need to compare
+// the row's own scalar/identity props, the PRESENCE of the dynamic node slots,
+// and the parent-computed renderSig snapshot. The comparator errs toward re-rendering:
+// any prop whose identity is unstable simply defeats the memo (no perf win) but
+// never causes staleness. Returns true to SKIP the re-render.
+//
+// The mutable-event vectors (edit / redaction / decryption / local-echo)
+// AND the children-only inputs (sender display name, urlPreview) are all
+// folded into the parent-computed `renderSig` prop (see getMessageRenderSig),
+// so they repaint correctly. `mediaAutoLoad` is mount-time (autoPlay /
+// initial fetch), so it needs no signature term.
+function areMessagePropsEqual(
+  prev: ComponentProps<typeof MessageInner>,
+  next: ComponentProps<typeof MessageInner>
+): boolean {
+  return (
+    prev.room === next.room &&
+    prev.mEvent === next.mEvent &&
+    prev.relations === next.relations &&
+    prev.collapse === next.collapse &&
+    prev.highlight === next.highlight &&
+    prev.edit === next.edit &&
+    prev.canDelete === next.canDelete &&
+    prev.canSendReaction === next.canSendReaction &&
+    prev.canPinEvent === next.canPinEvent &&
+    prev.imagePackRooms === next.imagePackRooms &&
+    prev.hideReadReceipts === next.hideReadReceipts &&
+    prev.showDeveloperTools === next.showDeveloperTools &&
+    prev.memberPowerTag === next.memberPowerTag &&
+    prev.accessibleTagColors === next.accessibleTagColors &&
+    prev.legacyUsernameColor === next.legacyUsernameColor &&
+    prev.streamRailStart === next.streamRailStart &&
+    prev.streamRailEnd === next.streamRailEnd &&
+    prev.hideThreadReplyAffordance === next.hideThreadReplyAffordance &&
+    prev.hideMainReplyAffordance === next.hideMainReplyAffordance &&
+    prev.msgType === next.msgType &&
+    prev.layout === next.layout &&
+    prev.channelHeaderInBubble === next.channelHeaderInBubble &&
+    prev.className === next.className &&
+    prev.onUserClick === next.onUserClick &&
+    prev.onUsernameClick === next.onUsernameClick &&
+    prev.onReplyClick === next.onReplyClick &&
+    prev.onReactionToggle === next.onReactionToggle &&
+    prev.onEditId === next.onEditId &&
+    Boolean(prev.reply) === Boolean(next.reply) &&
+    Boolean(prev.reactions) === Boolean(next.reactions) &&
+    Boolean(prev.threadSummary) === Boolean(next.threadSummary) &&
+    // `data-message-item` is the row's ABSOLUTE virtual-paginator index, queried
+    // by scrollToItem(). It shifts for existing rows when older history
+    // paginates in at the top, so a skipped render must not leave it stale —
+    // live appends at the bottom don't move it, so the memo still fires there.
+    (prev as Record<string, unknown>)['data-message-item'] ===
+      (next as Record<string, unknown>)['data-message-item'] &&
+    prev.renderSig === next.renderSig
+  );
+}
+
+export const Message = memo(MessageInner, areMessagePropsEqual);
+
 export type EventProps = {
   room: Room;
   mEvent: MatrixEvent;