diff --git a/frontend/libs/components/promptbox/src/lib/MentionTextarea.spec.tsx b/frontend/libs/components/promptbox/src/lib/MentionTextarea.spec.tsx
new file mode 100644
--- /dev/null
+++ b/frontend/libs/components/promptbox/src/lib/MentionTextarea.spec.tsx
@@ -0,0 +1,109 @@
+import { act, createElement } from "react";
+import { createRoot } from "react-dom/client";
+import { MentionTextarea } from "./MentionTextarea";
+
+// These subtrees are not opened by the tests; keep the real editable and
+// native React composition/keyboard event processing under test.
+vi.mock("./MentionChipMenu", () => ({ MentionChipMenu: () => null }));
+vi.mock("./deck/DeckCard", () => ({ DeckPreviewModal: () => null }));
+vi.mock("./promptStore", () => ({
+  useEnterToGenerateStore: (select: (s: { enabled: boolean }) => unknown) => select({ enabled: false }),
+}));
+
+let container: HTMLDivElement;
+let root: ReturnType<typeof createRoot>;
+let execCommand: ReturnType<typeof vi.fn>;
+
+beforeEach(() => {
+  vi.stubGlobal("IS_REACT_ACT_ENVIRONMENT", true);
+  vi.stubGlobal("requestAnimationFrame", (cb: FrameRequestCallback) => setTimeout(() => cb(0), 0));
+  // jsdom has no layout scrolling; the dropdown itself stays real.
+  Object.defineProperty(HTMLElement.prototype, "scrollIntoView", { value: vi.fn(), configurable: true });
+  execCommand = vi.fn(() => true);
+  Object.defineProperty(document, "execCommand", { value: execCommand, configurable: true });
+  container = document.createElement("div");
+  document.body.appendChild(container);
+  root = createRoot(container);
+});
+
+afterEach(() => {
+  act(() => root.unmount());
+  container.remove();
+  Reflect.deleteProperty(HTMLElement.prototype, "scrollIntoView");
+  vi.unstubAllGlobals();
+});
+
+it.each([false, true])("does not edit or submit while composing (enterToGenerate=%s)", (enterToGenerate) => {
+  const onKeyDown = vi.fn();
+  const onChange = vi.fn();
+  act(() => root.render(createElement(MentionTextarea, {
+    value: "", mentionItems: [], colorMap: {}, onChange, onKeyDown, enterToGenerate,
+  })));
+  const editor = container.querySelector("[contenteditable]")!;
+  act(() => editor.dispatchEvent(new CompositionEvent("compositionstart", { bubbles: true, data: "候補" })));
+  const event = new KeyboardEvent("keydown", { key: "Enter", bubbles: true, cancelable: true });
+  act(() => editor.dispatchEvent(event));
+  expect(event.defaultPrevented).toBe(false);
+  expect(execCommand).not.toHaveBeenCalled();
+  expect(onKeyDown).not.toHaveBeenCalled();
+  expect(onChange).not.toHaveBeenCalled();
+});
+
+it.each([{ isComposing: true }, { keyCode: 229 }])("recognizes native IME keyboard flags %j", (flags) => {
+  const onKeyDown = vi.fn();
+  act(() => root.render(createElement(MentionTextarea, {
+    value: "", mentionItems: [], colorMap: {}, onChange: vi.fn(), onKeyDown, enterToGenerate: true,
+  })));
+  const editor = container.querySelector("[contenteditable]")!;
+  const event = new KeyboardEvent("keydown", { key: "Enter", ...flags, bubbles: true, cancelable: true });
+  act(() => editor.dispatchEvent(event));
+  expect(event.defaultPrevented).toBe(false);
+  expect(onKeyDown).not.toHaveBeenCalled();
+});
+
+it("still submits ordinary Enter after composition ends", () => {
+  const onKeyDown = vi.fn();
+  act(() => root.render(createElement(MentionTextarea, {
+    value: "", mentionItems: [], colorMap: {}, onChange: vi.fn(), onKeyDown, enterToGenerate: true,
+  })));
+  const editor = container.querySelector("[contenteditable]")!;
+  act(() => editor.dispatchEvent(new CompositionEvent("compositionstart", { bubbles: true })));
+  act(() => editor.dispatchEvent(new CompositionEvent("compositionend", { bubbles: true })));
+  act(() => editor.dispatchEvent(new KeyboardEvent("keydown", { key: "Enter", bubbles: true })));
+  expect(onKeyDown).toHaveBeenCalledOnce();
+});
+
+it("still inserts a newline for ordinary Enter when generation is disabled", () => {
+  act(() => root.render(createElement(MentionTextarea, {
+    value: "", mentionItems: [], colorMap: {}, onChange: vi.fn(), enterToGenerate: false,
+  })));
+  const editor = container.querySelector("[contenteditable]")!;
+  act(() => editor.dispatchEvent(new KeyboardEvent("keydown", { key: "Enter", bubbles: true, cancelable: true })));
+  expect(execCommand).toHaveBeenCalledWith("insertLineBreak");
+});
+
+it.each(["Enter", "Tab"])("does not select an open mention during IME confirmation with %s", (key) => {
+  const onMentionSelect = vi.fn();
+  act(() => root.render(createElement(MentionTextarea, {
+    value: "", mentionItems: [{ label: "@Image1", type: "image" }], colorMap: {}, onChange: vi.fn(), onMentionSelect,
+  })));
+  const editor = container.querySelector("[contenteditable]")!;
+  act(() => {
+    editor.textContent = "@";
+    const range = document.createRange();
+    range.selectNodeContents(editor);
+    range.collapse(false);
+    const selection = window.getSelection()!;
+    selection.removeAllRanges();
+    selection.addRange(range);
+    editor.dispatchEvent(new InputEvent("input", { bubbles: true, inputType: "insertText", data: "@" }));
+  });
+  expect(document.querySelector("[data-mention-dropdown]")).not.toBeNull();
+  act(() => editor.dispatchEvent(new CompositionEvent("compositionstart", { bubbles: true })));
+  const event = new KeyboardEvent("keydown", { key, bubbles: true, cancelable: true });
+  act(() => editor.dispatchEvent(event));
+  expect(event.defaultPrevented).toBe(false);
+  expect(execCommand).not.toHaveBeenCalled();
+  expect(onMentionSelect).not.toHaveBeenCalled();
+  expect(document.querySelector("[data-mention-dropdown]")).not.toBeNull();
+});
diff --git a/frontend/libs/components/promptbox/src/lib/MentionTextarea.tsx b/frontend/libs/components/promptbox/src/lib/MentionTextarea.tsx
--- a/frontend/libs/components/promptbox/src/lib/MentionTextarea.tsx
+++ b/frontend/libs/components/promptbox/src/lib/MentionTextarea.tsx
@@ -1323,6 +1323,17 @@ export const MentionTextarea = forwardRef<HTMLDivElement, MentionTextareaProps>(
 
     const handleKeyDown = useCallback(
       (e: React.KeyboardEvent<HTMLDivElement>) => {
+        // IME confirmation keys belong to the composition session, not to
+        // mention selection, newline insertion or Enter-to-generate. Safari
+        // may clear isComposing on the final keydown but still reports 229.
+        if (
+          isComposing.current ||
+          e.nativeEvent.isComposing ||
+          e.nativeEvent.keyCode === 229
+        ) {
+          return;
+        }
+
         if (mentionState.isOpen && filteredItems.length > 0) {
           if (e.key === "ArrowDown") {
             e.preventDefault();