fix(a11y): stable reaction labels — emoji glyph, custom emoji shortcode (#179)

The reaction button's aria-label used getShortcodeFor(), which returns
undefined until the lazily loaded emoji data arrives. The same button read
"🎉 reaction, 1 person" on first render and "tada reaction, 2 people" after
any later re-render. It now always uses the emoji itself (screen readers
speak it by its proper name, e.g. "party popper").

Custom (mxc) emoji were labelled just "custom emoji"; they now use the
shortcode carried on the reaction event (":lotus_blob: reaction").

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/components/message/Reaction.tsx

@@ -13,13 +13,21 @@ export const Reaction = as<
     mx: MatrixClient;
     count: number;
     reaction: string;
+    /** Custom-emoji shortcode from the reaction event, used for its label. */
+    shortcode?: string;
     useAuthentication?: boolean;
   }
->(({ className, mx, count, reaction, useAuthentication, ...props }, ref) => {
-  const shortcode = reaction.startsWith('mxc://')
-    ? 'custom emoji'
-    : (getShortcodeFor(getHexcodeForEmoji(reaction)) ?? reaction);
-  const label = `${shortcode} reaction, ${count} ${count === 1 ? 'person' : 'people'}`;
+>(({ className, mx, count, reaction, shortcode, useAuthentication, ...props }, ref) => {
+  // Name a unicode reaction by the emoji itself: screen readers speak it by its
+  // proper name ("party popper"). Don't use getShortcodeFor here — it returns
+  // undefined until the lazy emoji data loads, so the same button would flip
+  // between "🎉 reaction" and "tada reaction" depending on timing (#179).
+  const name = reaction.startsWith('mxc://')
+    ? shortcode
+      ? `:${shortcode}:`
+      : 'custom emoji'
+    : reaction;
+  const label = `${name} reaction, ${count} ${count === 1 ? 'person' : 'people'}`;
 
   return (
     <Box

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

@@ -23,7 +23,7 @@ import { Room } from 'matrix-js-sdk';
 import { type Relations } from 'matrix-js-sdk/lib/models/relations';
 import FocusTrap from 'focus-trap-react';
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
-import { factoryEventSentBy } from '../../../utils/matrix';
+import { eventWithShortcode, factoryEventSentBy } from '../../../utils/matrix';
 import { Reaction, ReactionTooltipMsg } from '../../../components/message';
 import { useRelations } from '../../../hooks/useRelations';
 import * as css from './styles.css';
@@ -138,6 +138,7 @@ export const Reactions = as<'div', ReactionsProps>(
               key={key}
               mx={mx}
               reaction={key}
+              shortcode={rEvents.find(eventWithShortcode)?.getContent().shortcode}
               count={events.size}
               onClick={canSendReaction ? () => onReactionToggle(mEventId, key) : undefined}
               onContextMenu={handleViewReaction}

src/app/features/room/reaction-viewer/ReactionViewer.tsx

@@ -107,6 +107,7 @@ export const ReactionViewer = as<'div', ReactionViewerProps>(
                     key={key}
                     mx={mx}
                     reaction={key}
+                    shortcode={Array.from(evts).find(eventWithShortcode)?.getContent().shortcode}
                     count={evts.size}
                     role="option"
                     aria-selected={key === selectedKey}