fix(composer): stop the intermittent autocomplete-insert crash

Picking an autocomplete item (mention/emoji/command — all inline voids)
occasionally tripped the composer error boundary, forcing a page refresh, even
though the element had already inserted. Root cause (traced through slate-react):
moveCursor deferred its cursor work to setTimeout(0), leaving the caret on the
just-inserted void's zero-width edge whose DOM (a U+FEFF node) isn't populated on
that tick. slate-react's commit-phase selection sync then calls
setBaseAndExtent(voidEdge, 1) and throws IndexSizeError mid-render → boundary.

Prevention: do the cursor work SYNCHRONOUSLY, in the same commit as the insert —
Transforms.move (escapes the void into the real trailing text node) then
insertText(' '). The caret is then always a resolvable text point when the
selection sync runs. (moveCursor's focus stays deferred+guarded, unchanged.)

Recovery (belt-and-suspenders): the composer error boundary is now recoverable —
a "Reload composer" button (resetErrorBoundary) + onReset Transforms.deselect
clears a transient bad selection so it remounts with the draft intact, no page
refresh. + role="alert" for screen readers.

Three review agents: two root-caused the exact slate-react throw and proved the
try/catch-only version merely recovered; a third reproduced the transforms
headlessly and caught that a first "sync insertText WITHOUT move" attempt hit
Slate's void guard (space dropped, caret trapped) — the move is required to
escape the void. Not unit-testable (needs the live DOM + the timing race).
Gate-green (tsc, eslint, prettier, 925 tests, build).

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

src/app/components/editor/utils.ts

@@ -194,22 +194,43 @@ export const createCommandElement = (command: string): CommandElement => ({
 });
 
 export const replaceWithElement = (editor: Editor, selectRange: BaseRange, element: Element) => {
-  Transforms.select(editor, selectRange);
-  Transforms.insertNodes(editor, element);
-  Transforms.collapse(editor, {
-    edge: 'end',
-  });
+  // Wrap the whole sequence: on a stale autocomplete range (the document changed
+  // between the menu opening and the pick) `insertNodes` — not `select`, which is
+  // lazy in this Slate version — can throw. This runs inside the pick's event
+  // handler, so an escape wouldn't hit the error boundary, but keep it contained.
+  try {
+    Transforms.select(editor, selectRange);
+    Transforms.insertNodes(editor, element);
+    Transforms.collapse(editor, { edge: 'end' });
+  } catch {
+    /* stale range — the pick is a no-op rather than an uncaught error */
+  }
 };
 
 export const moveCursor = (editor: Editor, withSpace?: boolean) => {
-  // Defer to the next tick so React can flush any pending void-element DOM
-  // updates (e.g. after inserting a mention) before Slate resolves cursor
-  // positions via ReactEditor.toDOMNode — otherwise Slate throws
-  // "Cannot resolve a DOM node from slate node".
+  // Move the caret out of the just-inserted inline void and land it in a real
+  // trailing text node — SYNCHRONOUSLY, in the same commit as the insert.
+  // `Transforms.move` escapes the void (after insertNodes+collapse the caret is
+  // INSIDE the void's inner text node; insertText there is a no-op, blocked by
+  // Slate's void guard). The space then lands in a real text node.
+  // Doing this in the same commit (vs the old deferred setTimeout) means the
+  // caret never sits on the void's zero-width edge on a racy tick — that edge's
+  // DOM (a U+FEFF node) isn't populated yet, so slate-react's commit-phase
+  // selection sync (setBaseAndExtent) threw IndexSizeError mid-render and tripped
+  // the composer error boundary. Both ops are pure model transforms (no DOM
+  // resolution), so running them synchronously is safe.
+  Transforms.move(editor);
+  if (withSpace) editor.insertText(' ');
+  // Re-assert focus next tick (a pick usually keeps the editor focused). Guarded
+  // because ReactEditor.focus resolves the DOM; with the caret now in a real text
+  // node this is safe, but stay defensive against a mid-flight editor.
   setTimeout(() => {
-    ReactEditor.focus(editor);
-    Transforms.move(editor);
-    if (withSpace) editor.insertText(' ');
+    try {
+      ReactEditor.focus(editor);
+    } catch {
+      // The editor DOM can be mid-flight (autocomplete just closed / re-render
+      // landed). The element is already inserted, so skip the focus nudge.
+    }
   }, 0);
 };
 

src/app/features/room/RoomView.tsx

@@ -1,8 +1,9 @@
 import React, { useCallback, useMemo, useRef } from 'react';
 import { ErrorBoundary } from 'react-error-boundary';
-import { Box, Text, config } from 'folds';
+import { Box, Button, Text, config } from 'folds';
 import { EventType } from 'matrix-js-sdk';
 import { ReactEditor } from 'slate-react';
+import { Transforms } from 'slate';
 import { isKeyHotkey } from 'is-hotkey';
 import { useStateEvent } from '../../hooks/useStateEvent';
 import { StateEvent } from '../../../types/matrix/room';
@@ -152,17 +153,38 @@ export function RoomView({ eventId }: { eventId?: string }) {
             <>
               {canMessage && (
                 <ErrorBoundary
-                  fallback={
+                  onReset={() => {
+                    // The composer crash is a transient bad-selection render
+                    // (e.g. after an autocomplete insert); the draft content is
+                    // intact. Clear the selection so the remounted composer can
+                    // render — the user clicks in to continue, no page refresh.
+                    try {
+                      Transforms.deselect(editor);
+                    } catch {
+                      /* editor already in a safe state */
+                    }
+                  }}
+                  fallbackRender={({ resetErrorBoundary }) => (
                     <RoomInputPlaceholder
+                      role="alert"
                       style={{ padding: config.space.S200 }}
+                      direction="Column"
                       alignItems="Center"
                       justifyContent="Center"
+                      gap="200"
                     >
-                      <Text align="Center">
-                        Message composer encountered an error. Try refreshing.
-                      </Text>
+                      <Text align="Center">The message composer hit a snag.</Text>
+                      <Button
+                        size="300"
+                        variant="Secondary"
+                        fill="Soft"
+                        radii="300"
+                        onClick={resetErrorBoundary}
+                      >
+                        <Text size="B300">Reload composer</Text>
+                      </Button>
                     </RoomInputPlaceholder>
-                  }
+                  )}
                 >
                   <RoomInput
                     room={room}