feat(a11y): message semantics for screen readers (P3-4)

- Each message is role="article"; collapsed messages (consecutive from one
  sender) now carry an aria-label with sender + time — previously a screen
  reader heard only the body with no attribution (the biggest a11y gap).
  Pure messageAriaLabel() reuses the existing time utils (+3 tests).
- Editing a message announces "Editing message from <sender>" (ariaLabel
  threaded MessageEditor → CustomEditor; the main composer is unaffected).
- System emoji get role="img" + aria-label from the shortcode; custom
  emoticons always have an accessible name.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/components/editor/Editor.tsx

@@ -66,6 +66,8 @@ type CustomEditorProps = {
   maxHeight?: string;
   editor: Editor;
   placeholder?: string;
+  /** Explicit accessible name for the textbox; falls back to the placeholder. */
+  ariaLabel?: string;
   onKeyDown?: KeyboardEventHandler;
   onKeyUp?: KeyboardEventHandler;
   onChange?: EditorChangeHandler;
@@ -82,6 +84,7 @@ export const CustomEditor = forwardRef<HTMLDivElement, CustomEditorProps>(
       maxHeight = '50vh',
       editor,
       placeholder,
+      ariaLabel,
       onKeyDown,
       onKeyUp,
       onChange,
@@ -139,7 +142,7 @@ export const CustomEditor = forwardRef<HTMLDivElement, CustomEditorProps>(
                 data-editable-name={editableName}
                 className={css.EditorTextarea}
                 placeholder={placeholder}
-                aria-label={placeholder ?? 'Message input'}
+                aria-label={ariaLabel ?? placeholder ?? 'Message input'}
                 aria-multiline="true"
                 renderPlaceholder={renderPlaceholder}
                 renderElement={renderElement}

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

@@ -56,6 +56,7 @@ import {
   getMemberDisplayName,
 } from '../../../utils/room';
 import { getMxIdLocalPart, mxcUrlToHttp } from '../../../utils/matrix';
+import { messageAriaLabel } from '../../../utils/a11y';
 import { MessageLayout, MessageSpacing } from '../../../state/settings';
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
 import { useRecentEmoji } from '../../../hooks/useRecentEmoji';
@@ -972,6 +973,10 @@ export const Message = React.memo(
             [MsgAppearClass]: playAppear,
             [MentionHighlightPulse]: playMentionPulse,
           })}
+          role="article"
+          aria-label={
+            collapse ? messageAriaLabel(senderDisplayName, mEvent.getTs(), hour24Clock) : undefined
+          }
           tabIndex={0}
           space={messageSpacing}
           collapse={collapse}

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

@@ -51,7 +51,13 @@ import { UseStateProvider } from '../../../components/UseStateProvider';
 import { EmojiBoard } from '../../../components/emoji-board';
 import { AsyncStatus, useAsyncCallback } from '../../../hooks/useAsyncCallback';
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
-import { getEditedEvent, getMentionContent, trimReplyFromFormattedBody } from '../../../utils/room';
+import {
+  getEditedEvent,
+  getMemberDisplayName,
+  getMentionContent,
+  trimReplyFromFormattedBody,
+} from '../../../utils/room';
+import { getMxIdLocalPart } from '../../../utils/matrix';
 import { mobileOrTablet } from '../../../utils/user-agent';
 import { useComposingCheck } from '../../../hooks/useComposingCheck';
 
@@ -66,6 +72,12 @@ export const MessageEditor = as<'div', MessageEditorProps>(
   ({ room, roomId, mEvent, imagePackRooms, onCancel, ...props }, ref) => {
     const mx = useMatrixClient();
     const editor = useEditor();
+    // Accessible name for the edit textbox so screen readers announce which
+    // message is being edited (a11y, P3-4).
+    const editSenderId = mEvent.getSender();
+    const editSenderName = editSenderId
+      ? (getMemberDisplayName(room, editSenderId) ?? getMxIdLocalPart(editSenderId) ?? editSenderId)
+      : '';
     const [enterForNewline] = useSetting(settingsAtom, 'enterForNewline');
     const [globalToolbar] = useSetting(settingsAtom, 'editorToolbar');
     const [isMarkdown] = useSetting(settingsAtom, 'isMarkdown');
@@ -259,6 +271,7 @@ export const MessageEditor = as<'div', MessageEditorProps>(
         <CustomEditor
           editor={editor}
           placeholder="Edit message..."
+          ariaLabel={editSenderId ? `Editing message from ${editSenderName}` : 'Edit message'}
           onKeyDown={handleKeyDown}
           onKeyUp={handleKeyUp}
           bottom={

src/app/plugins/react-custom-html-parser.tsx

@@ -229,13 +229,21 @@ export const scaleSystemEmoji = (text: string): (string | JSX.Element)[] =>
   findAndReplace(
     text,
     EMOJI_REG_G,
-    (match, pushIndex) => (
-      <span key={`scaleSystemEmoji-${pushIndex}`} className={css.EmoticonBase}>
-        <span className={css.Emoticon()} title={getShortcodeFor(getHexcodeForEmoji(match[0]))}>
-          {match[0]}
+    (match, pushIndex) => {
+      const shortcode = getShortcodeFor(getHexcodeForEmoji(match[0]));
+      return (
+        <span key={`scaleSystemEmoji-${pushIndex}`} className={css.EmoticonBase}>
+          <span
+            className={css.Emoticon()}
+            title={shortcode}
+            aria-label={shortcode || undefined}
+            role={shortcode ? 'img' : undefined}
+          >
+            {match[0]}
+          </span>
         </span>
-      </span>
-    ),
+      );
+    },
     (txt) => txt,
   );
 
@@ -574,10 +582,20 @@ export const getReactCustomHtmlParser = (
             );
           }
           if (htmlSrc && 'data-mx-emoticon' in props) {
+            const emoticonAlt =
+              (typeof props.alt === 'string' && props.alt) ||
+              (typeof props.title === 'string' && props.title) ||
+              'emoji';
             return (
               <span className={css.EmoticonBase}>
                 <span className={css.Emoticon()}>
-                  <img {...props} className={css.EmoticonImg} src={htmlSrc} loading="lazy" />
+                  <img
+                    {...props}
+                    alt={emoticonAlt}
+                    className={css.EmoticonImg}
+                    src={htmlSrc}
+                    loading="lazy"
+                  />
                 </span>
               </span>
             );
@@ -611,7 +629,6 @@ export const getReactCustomHtmlParser = (
               <>
                 {segments.map((segment, index) => {
                   if (segment.type === 'text') {
-                    // eslint-disable-next-line react/no-array-index-key
                     return (
                       <React.Fragment key={index}>{renderTextChunk(segment.value)}</React.Fragment>
                     );
@@ -619,7 +636,6 @@ export const getReactCustomHtmlParser = (
                   const raw =
                     segment.type === 'block' ? `$$${segment.value}$$` : `$${segment.value}$`;
                   return (
-                    // eslint-disable-next-line react/no-array-index-key
                     <React.Fragment key={index}>
                       {renderMath(segment.value, segment.type === 'block', raw, raw)}
                     </React.Fragment>

src/app/utils/a11y.test.ts

@@ -0,0 +1,28 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import dayjs from 'dayjs';
+import { messageAriaLabel } from './a11y';
+import { timeDayMonthYear, timeHourMinute } from './time';
+
+test('messageAriaLabel composes sender, date and time (24h)', () => {
+  const ts = dayjs('2026-07-01T14:30:00').valueOf();
+  assert.equal(
+    messageAriaLabel('Alice', ts, true),
+    `Alice, ${timeDayMonthYear(ts)} ${timeHourMinute(ts, true)}`,
+  );
+});
+
+test('messageAriaLabel honours the 12-hour clock preference', () => {
+  const ts = dayjs('2026-07-01T14:30:00').valueOf();
+  assert.equal(
+    messageAriaLabel('Bob', ts, false),
+    `Bob, ${timeDayMonthYear(ts)} ${timeHourMinute(ts, false)}`,
+  );
+});
+
+test('messageAriaLabel keeps the sender name verbatim as plain text', () => {
+  const ts = dayjs('2026-07-01T09:05:00').valueOf();
+  const label = messageAriaLabel('@user:example.org', ts, true);
+  assert.ok(label.startsWith('@user:example.org, '));
+  assert.ok(!label.includes('<'));
+});

src/app/utils/a11y.ts

@@ -0,0 +1,14 @@
+import { timeDayMonthYear, timeHourMinute } from './time';
+
+/**
+ * Builds a plain-text accessible label for a message row, used when the
+ * visible sender/timestamp header is collapsed and therefore hidden from
+ * assistive technology.
+ *
+ * @param sender - Sender display name (already resolved to a human string).
+ * @param ts - Message origin timestamp in milliseconds.
+ * @param hour24Clock - Whether to format the time using a 24-hour clock.
+ * @returns A label such as `Alice, 1 July 2026 14:30`.
+ */
+export const messageAriaLabel = (sender: string, ts: number, hour24Clock: boolean): string =>
+  `${sender}, ${timeDayMonthYear(ts)} ${timeHourMinute(ts, hour24Clock)}`;