fix(webchat): skip textarea resize during IME composition to eliminate typing lag

ui/src/ui/views/chat.test.ts

@@ -1839,6 +1839,33 @@ describe("chat composer IME composition", () => {
     expect(onSend).not.toHaveBeenCalled();
     expect(onHistoryKeydown).not.toHaveBeenCalled();
   });
+
+  it("does not force textarea resize during IME composition", () => {
+    const container = renderChatView({});
+    const textarea = requireElement(
+      container,
+      ".agent-chat__composer-combobox > textarea",
+      "composer textarea",
+    ) as HTMLTextAreaElement;
+
+    // Set a sentinel height to detect unwanted overwrites
+    textarea.style.height = "42px";
+
+    textarea.dispatchEvent(new CompositionEvent("compositionstart", { bubbles: true }));
+    textarea.value = "shi";
+    textarea.dispatchEvent(new InputEvent("input", { bubbles: true, isComposing: true }));
+    textarea.value = "shichang";
+    textarea.dispatchEvent(new InputEvent("input", { bubbles: true, isComposing: true }));
+
+    // Height must stay untouched — no forced reflow during composition
+    expect(textarea.style.height).toBe("42px");
+
+    textarea.value = "市场";
+    textarea.dispatchEvent(new CompositionEvent("compositionend", { bubbles: true }));
+
+    // After composition ends, adjustTextareaHeight runs via syncComposerValue
+    expect(textarea.style.height).not.toBe("42px");
+  });
 });
 
 describe("chat slash menu accessibility", () => {

ui/src/ui/views/chat.ts

@@ -2422,7 +2422,10 @@ export function renderChat(props: ChatProps) {
   const handleInput = (e: InputEvent) => {
     const target = e.target as HTMLTextAreaElement;
     if (vs.composerComposing || e.isComposing) {
-      adjustTextareaHeight(target);
+      // Skip adjustTextareaHeight during IME composition — each pinyin
+      // keystroke fires `input` and the height read/write forces a
+      // synchronous reflow that blocks the composition thread.
+      // Resize runs once in handleCompositionEnd → syncComposerValue.
       draftMirror.value = target.value;
       return;
     }