fix(transcription): cancel pending @mention debounce in onExit

Without this, a closed dropdown's trailing runSearch could fire against
the next dropdown's state and silently overwrite its items before its
own fetch resolved. Felix #1 on PR #629.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/shared/discussion/PersonMentionEditor.svelte

@@ -268,6 +268,13 @@ onMount(() => {
 								return exports?.onKeyDown(event) ?? false;
 							},
 							onExit() {
+								// Cancel any pending debounce so a closed dropdown's trailing
+								// runSearch cannot fire against the *next* dropdown's state.
+								// The hoisted `cancelPendingSearch` would be overwritten by
+								// the next render()'s onStart before the trailing call fires,
+								// so we cancel locally via the closure-scoped debouncedSearch.
+								// Felix #1 on PR #629.
+								debouncedSearch.cancel();
 								if (mountedDropdown) {
 									unmount(mountedDropdown);
 									mountedDropdown = null;

frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts

@@ -309,6 +309,46 @@ describe('PersonMentionEditor — stale-response race', () => {
 	});
 });
 
+// ─── onExit cancels pending debounce (Felix #1 on PR #629) ───────────────────
+
+describe('PersonMentionEditor — onExit cancels pending debounce', () => {
+	it('cancels the pending debounced fetch when Escape closes the dropdown before the debounce fires', async () => {
+		const fetchMock = vi.fn().mockResolvedValue({ ok: true, json: vi.fn().mockResolvedValue([]) });
+		vi.stubGlobal('fetch', fetchMock);
+		renderHost();
+
+		// Open the dropdown by typing @ + a query in the editor.
+		await userEvent.type(page.getByRole('textbox'), '@A');
+		await vi.waitFor(async () => {
+			await expect.element(page.getByRole('searchbox')).toBeVisible();
+		});
+
+		// Wait for any in-flight fetch from opening the dropdown to settle.
+		await new Promise((r) => setTimeout(r, SEARCH_DEBOUNCE_MS + POST_DEBOUNCE_SLACK_MS));
+		const fetchesBeforeEscape = fetchMock.mock.calls.length;
+
+		// Trigger a new debounced search (queues runSearch after 150 ms), then
+		// immediately Escape *while focus is back in the editor* so Tiptap's
+		// suggestion-plugin Escape handler fires onExit before the debounce.
+		// Without onExit cancelling the pending debounce, runSearch executes
+		// against the now-unmounted dropdown's state.
+		await page.getByRole('searchbox').fill('Walter');
+		// Focus the editor so the Escape lands on Tiptap's suggestion handler.
+		(page.getByRole('textbox').element() as HTMLElement).focus();
+		await userEvent.keyboard('{Escape}');
+
+		// Wait past the debounce window. If onExit did not cancel the pending
+		// debounce, a fetch with q=Walter would still fire here.
+		await new Promise((r) => setTimeout(r, SEARCH_DEBOUNCE_MS + POST_DEBOUNCE_SLACK_MS));
+
+		const newFetches = fetchMock.mock.calls.slice(fetchesBeforeEscape);
+		const walterFetches = newFetches.filter(
+			([url]) => typeof url === 'string' && url.includes('q=Walter')
+		);
+		expect(walterFetches.length).toBe(0);
+	});
+});
+
 // ─── AC-1: search input prefilled with text typed after @ ───────────────────
 
 describe('PersonMentionEditor — AC-1: search input prefill', () => {