diff --git a/changes/sync-ui-window-close-quit.md b/changes/sync-ui-window-close-quit.md
new file mode 100644
index 00000000..26726f5c
--- /dev/null
+++ b/changes/sync-ui-window-close-quit.md
@@ -0,0 +1,5 @@
+type: fixed
+area: sync
+
+- Fixed the app staying alive in the background after closing a standalone Sync window (`subminer sync --ui`) on macOS. The quit re-issued after async will-quit cleanup was dropped by Electron when the cleanup settled within the same tick as the will-quit dispatch; the re-quit now runs on a fresh macrotask, and the standalone Sync window close path uses the same forced-exit fallback as SIGTERM. Windows opened from the tray menu of a running app are unaffected: closing them still leaves the app running.
+- Fixed cancelling or timing out a sync run hanging when the sync child had already exited without emitting a terminal result: the run waited on `close`, which a descendant holding the inherited stdio pipes can delay indefinitely, stalling the quit-time sync shutdown. Cancellation now settles the run immediately once the child has exited.
diff --git a/src/core/services/app-lifecycle.test.ts b/src/core/services/app-lifecycle.test.ts
index bba2d4e4..9d6eb74a 100644
--- a/src/core/services/app-lifecycle.test.ts
+++ b/src/core/services/app-lifecycle.test.ts
@@ -168,7 +168,14 @@ test('startAppLifecycle defers quit until async cleanup settles', async () => {
   assert.ok(releaseCleanup);
   (releaseCleanup as () => void)();
   await cleanupDone;
+  // The re-quit must not fire in the microtask turn of the will-quit
+  // dispatch: Electron drops a quit issued while the prevented quit is
+  // still unwinding, leaving a windowless process alive.
   await Promise.resolve();
+  await Promise.resolve();
+  await Promise.resolve();
+  assert.deepEqual(calls, []);
+  await new Promise((resolve) => setImmediate(resolve));
   assert.deepEqual(calls, ['quitApp']);
 });
 
diff --git a/src/core/services/app-lifecycle.ts b/src/core/services/app-lifecycle.ts
index e74643cd..e71fc138 100644
--- a/src/core/services/app-lifecycle.ts
+++ b/src/core/services/app-lifecycle.ts
@@ -219,7 +219,11 @@ export function startAppLifecycle(initialArgs: CliArgs, deps: AppLifecycleServic
       .finally(() => {
         quitCleanupPending = false;
         quitCleanupComplete = true;
-        deps.quitApp();
+        // A cleanup promise that settles in a microtask would re-quit while
+        // Electron is still unwinding the prevented quit, and that quit call
+        // is silently dropped, leaving a windowless process alive. Re-issue
+        // the quit from a fresh macrotask instead.
+        setImmediate(() => deps.quitApp());
       });
   });
 
diff --git a/src/main.ts b/src/main.ts
index c8226f6a..844aa648 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -2247,7 +2247,9 @@ const openSyncUiWindowHandler = createOpenConfigSettingsWindowHandler({
   promoteSettingsWindowAboveOverlay: (window) =>
     promoteSettingsWindowAboveOverlay(window as BrowserWindow),
   onClosed: () => {
-    if (appState.initialArgs?.syncWindow) app.quit();
+    // requestAppQuit (not app.quit) so the forced-exit fallback covers a
+    // stalled quit; a standalone sync window is the app's only reason to live.
+    if (appState.initialArgs?.syncWindow) requestAppQuit();
   },
   log: (message) => logger.error(message),
 });
diff --git a/src/main/runtime/sync-launcher-client.test.ts b/src/main/runtime/sync-launcher-client.test.ts
index 7e72998e..d326f71e 100644
--- a/src/main/runtime/sync-launcher-client.test.ts
+++ b/src/main/runtime/sync-launcher-client.test.ts
@@ -165,6 +165,56 @@ test('runSyncLauncher cancel kills the child and resolves as cancelled', async (
   assert.match(result.error ?? '', /cancel/i);
 });
 
+test('runSyncLauncher cancel settles a child that already exited without a result', async () => {
+  const { spawn, children } = makeSpawn();
+  const handle = runSyncLauncher({
+    command: ['subminer'],
+    args: ['sync', 'media-box', '--json'],
+    onEvent: () => {},
+    spawn,
+  });
+  const child = children[0]!;
+  // The child is gone, but a descendant still holds the inherited stdio pipes,
+  // so `close` never arrives.
+  child.kill = () => {
+    child.killed = true;
+    return true;
+  };
+  child.emit('exit', null, 'SIGTERM');
+
+  handle.cancel();
+
+  const result = await Promise.race([
+    handle.done,
+    new Promise<null>((resolve) => setTimeout(() => resolve(null), 25)),
+  ]);
+  assert.deepEqual(result, { ok: false, error: 'Sync cancelled.' });
+  assert.equal(child.killed, false);
+});
+
+test('runSyncLauncher times out a child that already exited without a result', async () => {
+  const { spawn, children } = makeSpawn();
+  const handle = runSyncLauncher({
+    command: ['subminer'],
+    args: ['sync', 'media-box', '--check', '--json'],
+    onEvent: () => {},
+    spawn,
+    timeoutMs: 1,
+  });
+  const child = children[0]!;
+  child.kill = () => {
+    child.killed = true;
+    return true;
+  };
+  child.emit('exit', null, 'SIGTERM');
+
+  const result = await Promise.race([
+    handle.done,
+    new Promise<null>((resolve) => setTimeout(() => resolve(null), 25)),
+  ]);
+  assert.deepEqual(result, { ok: false, error: 'Sync operation timed out.' });
+});
+
 test('runSyncLauncher times out a child that never completes', async () => {
   const { spawn, children } = makeSpawn();
   const handle = runSyncLauncher({
diff --git a/src/main/runtime/sync-launcher-client.ts b/src/main/runtime/sync-launcher-client.ts
index ef060721..9f462ed0 100644
--- a/src/main/runtime/sync-launcher-client.ts
+++ b/src/main/runtime/sync-launcher-client.ts
@@ -75,6 +75,7 @@ export function runSyncLauncher(options: {
   let resultEvent: Extract<SyncProgressEvent, { type: 'result' }> | null = null;
   let terminationError: string | null = null;
   let settleAfterTerminalEvent: (() => void) | null = null;
+  let settleAfterTermination: (() => boolean) | null = null;
 
   child.stdout?.on('data', (chunk) => {
     stdoutBuffer += chunk.toString();
@@ -137,6 +138,11 @@ export function runSyncLauncher(options: {
     settleAfterTerminalEvent = () => {
       if (exitObserved) settleFromExit(exitCode);
     };
+    settleAfterTermination = () => {
+      if (!exitObserved) return false;
+      settleFromExit(exitCode);
+      return true;
+    };
     // Electron descendants can retain inherited stdio pipes after the main
     // child exits, delaying `close` indefinitely. Once terminal NDJSON has
     // arrived, `exit` is authoritative; keep `close` as the fallback.
@@ -153,6 +159,11 @@ export function runSyncLauncher(options: {
   const terminate = (error: string): void => {
     if (terminationError) return;
     terminationError = error;
+    // A child that already exited without terminal NDJSON is still waiting on
+    // `close`, which descendants holding inherited pipes can delay
+    // indefinitely. There is nothing left to signal, so settle now instead of
+    // leaving `done` (and the quit-time shutdown that awaits it) pending.
+    if (settleAfterTermination?.()) return;
     killTimer = setTimeout(() => {
       killTimer = null;
       try {