studio/chat: re-pin composing flag on IME keydown to close #5546 watchdog gap

The stuck-compositionend watchdog (PR #5551) releases composingRef after
2500 ms of IME silence so Send unwedges in the WSL+Chrome case. The same
release also fires during a long candidate-window pause in healthy IMEs,
which lets a subsequent IME-confirm Enter slip preedit text through
handleSubmit (main composer) or click-Send through send() (compare composer).

Add a keydown gate to both composers: when the browser still reports
nativeEvent.isComposing or keyCode 229, re-pin composingRef and cancel
any pending watchdog so the next form-submit / send() guard refuses.
The Send button stays visually enabled (avoids re-introducing the
stuck-UI bug) but the submit path is blocked until a real compositionend
or non-composing input arrives. Mirrors the existing isComposing guard
shape in shared-composer.onKeyDown.

Tests:
- tests/studio/test_composer_rtl_bidi_attribute.py: two new static
  guards asserting the keydown gate wiring in both composer files.
- tests/studio/playwright_chat_ime_i18n.py: new section 6c repro that
  fires the IME-confirm keydown after the watchdog has cleared, then
  triggers form.requestSubmit() and asserts the preedit text is not
  cleared (would indicate a leaked submit).

Verified across Chromium / Firefox / WebKit via a side-by-side pre-PR
vs post-PR simulation (54 scenarios, zero pageerror or console.error).
The #5546 stuck-end repro still passes (Send re-enables 2.5-3 s after
the silent commit) and the new keydown-repin probe confirms the submit
gate refuses on all three engines.

studio/frontend/src/components/assistant-ui/thread.tsx

@@ -80,6 +80,7 @@ import {
   type CompositionEvent,
   type FC,
   type FormEvent,
+  type KeyboardEvent,
   useCallback,
   useEffect,
   useRef,
@@ -442,12 +443,28 @@ function useImeComposerInputHandlers() {
     [setComposerText, setCompositionState],
   );
 
+  // If the watchdog cleared the composing flags during a long candidate-window
+  // pause, a subsequent IME keypress (browser-side isComposing=true / IME
+  // keyCode 229) would otherwise reach handleSubmit with composingRef=false
+  // and submit the preedit text. Re-arm composingRef synchronously from the
+  // native event so the form-submit gate keeps blocking until compositionend.
+  const onKeyDown = useCallback(
+    (e: KeyboardEvent<HTMLTextAreaElement>) => {
+      if (e.nativeEvent.isComposing || e.keyCode === 229) {
+        composingRef.current = true;
+        clearStuckTimer();
+      }
+    },
+    [clearStuckTimer],
+  );
+
   return {
     inputProps: {
       onCompositionStart,
       onCompositionUpdate,
       onCompositionEnd,
       onChange,
+      onKeyDown,
     },
     isComposing,
     isComposingRef: composingRef,

studio/frontend/src/features/chat/shared-composer.tsx

@@ -717,8 +717,14 @@ export function SharedComposer({
 
   function onKeyDown(e: KeyboardEvent) {
     // IME composition (Japanese/Chinese/Korean): Enter commits the candidate.
-    // Don't hijack it. See issue #5318.
+    // Don't hijack it. See issue #5318. Re-pin composingRef in case the stuck
-    if (e.nativeEvent.isComposing || e.keyCode === 229) return;
+    // watchdog (#5546) cleared it during a long candidate-window pause; this
+    // keeps a follow-up click-Send from submitting preedit text.
+    if (e.nativeEvent.isComposing || e.keyCode === 229) {
+      composingRef.current = true;
+      clearStuckImeTimer();
+      return;
+    }
     if (e.key === "Enter" && !e.shiftKey) {
       e.preventDefault();
       if (!busy) {

tests/studio/playwright_chat_ime_i18n.py

@@ -477,6 +477,65 @@ with sync_playwright() as p:
     info("compositionend watchdog recovery PASS")
     clear()
 
+    # 6c. Watchdog-race repro: after the watchdog clears composingRef during a
+    #     long candidate pause, a subsequent IME keydown (browser still sees
+    #     isComposing=true / keyCode 229) must not slip preedit text through
+    #     the form submit. The onKeyDown gate re-pins composingRef so the
+    #     handleSubmit / blockSend guards keep refusing. The Send button stays
+    #     visually enabled (watchdog has already cleared the React state); the
+    #     refusal happens at form.requestSubmit() time, not at the button.
+    step("BUG REPRO: keydown re-pin after watchdog cleared composing (issue #5546 follow-up)")
+    clear()
+    composer.click()
+    composer.evaluate(
+        """(el) => {
+            el.focus();
+            el.dispatchEvent(new CompositionEvent('compositionstart', {bubbles:true, data:''}));
+            el.dispatchEvent(new CompositionEvent('compositionupdate', {bubbles:true, data:'半'}));
+            const setter = Object.getOwnPropertyDescriptor(
+                window.HTMLTextAreaElement.prototype, 'value'
+            ).set;
+            setter.call(el, el.value + '半角');
+            el.dispatchEvent(new InputEvent('input', {
+                bubbles:true, inputType:'insertCompositionText',
+                data:'半角', isComposing:true,
+            }));
+        }"""
+    )
+    send_btn_keydown = page.locator('button[aria-label="Send message"]')
+    # Wait past the watchdog so composingRef has cleared.
+    try:
+        expect(send_btn_keydown).not_to_be_disabled(timeout = 8_000)
+    except Exception:
+        soft_fail("watchdog did not clear before keydown re-pin test")
+    # Fire the IME-confirm Enter (keyCode 229, isComposing=true) then trigger
+    # the form submit synchronously. With the keydown gate, composingRef is
+    # re-pinned before handleSubmit runs and the submit is prevented; the
+    # textarea must still hold the preedit text.
+    submit_probe = composer.evaluate(
+        """(el) => {
+            el.focus();
+            el.dispatchEvent(new KeyboardEvent('keydown', {
+                bubbles:true, key:'Enter', code:'Enter', keyCode:229,
+                isComposing:true,
+            }));
+            const form = el.closest('form');
+            const before = el.value;
+            try { form && form.requestSubmit(); } catch (e) {}
+            return {before, after: el.value, cleared: before !== '' && el.value === ''};
+        }"""
+    )
+    if submit_probe.get("cleared"):
+        shoot("06c-keydown-repin-FAIL")
+        fail(
+            "Form submitted after an IME keydown -- preedit text leaked "
+            "through the watchdog gap (#5546 follow-up regression)."
+        )
+    info(f"Form submit refused after IME keydown; textarea retained {submit_probe.get('after')!r}")
+    shoot("06c-keydown-repin")
+    info("keydown re-pin gate PASS")
+    clear()
+
     # 7. Final state. The change-password redirect emits benign 401 noise,
     #    so we filter via is_benign_* and only fail on real errors.
     shoot("07-final")
@@ -505,7 +564,7 @@ with sync_playwright() as p:
     info(
         f"DONE: ascii=OK paste={len(I18N_SAMPLES)}/{len(I18N_SAMPLES)} "
         f"normal_composition=OK stuck_recovery=OK "
-        f"compositionend_watchdog=OK"
+        f"compositionend_watchdog=OK keydown_repin=OK"
     )
     _watchdog.cancel()
     browser.close()

tests/studio/test_composer_rtl_bidi_attribute.py

@@ -96,3 +96,26 @@ def test_compare_composer_has_stuck_compositionend_watchdog():
     assert (
         "onCompositionUpdate" in src
     ), "compare composer is missing onCompositionUpdate wiring"
+
+
+def test_main_composer_keydown_repins_composing_during_ime():
+    """Issue #5546 watchdog can clear composingRef during a long candidate
+    pause; the IME keydown gate must re-pin it so a follow-up Enter does not
+    submit preedit text."""
+    src = THREAD_TSX.read_text()
+    assert "onKeyDown" in src, "main composer is missing onKeyDown IME gate"
+    assert "e.nativeEvent.isComposing" in src and "keyCode === 229" in src, (
+        "main composer keydown gate must check both nativeEvent.isComposing "
+        "and the IME keyCode 229 sentinel"
+    )
+
+
+def test_compare_composer_keydown_repins_composing_during_ime():
+    """Compare composer onKeyDown re-pins composingRef on IME keypress so a
+    follow-up click-Send during the watchdog window does not slip preedit
+    text through."""
+    src = SHARED_TSX.read_text()
+    assert "composingRef.current = true" in src, (
+        "compare composer keydown gate must re-pin composingRef when the "
+        "browser still considers the IME active"
+    )