[client] Serialize window operations while a slot is being created

Callers arriving after the window is published but before the creator
has drained the queue took the existing-window fast path and could run
ahead of older queued operations, so a newer SetURL could be overwritten
by an older one. withWindow now queues every caller while the creating
flag is set and clears the flag only once the queue is seen empty under
the lock.

A factory panic or a nil window left the creating flag set and the slot
dead; creation and drain now reset that state on early exit.

hideOtherWindows records the windows it hid only when no restore ran
in between, tracked by a generation counter, and re-shows them otherwise,
so a restore racing the hide cannot strand hidden windows.

client/ui/services/windowmanager_test.go

@@ -125,3 +125,73 @@ func TestWithWindowConcurrentCallersShareOneCreation(t *testing.T) {
 	require.Equal(t, int32(2), opCalls.Load())
 	require.NotNil(t, slot)
 }
+
+func TestWithWindowOpsQueuedDuringCreationRunInArrivalOrder(t *testing.T) {
+	s := newTestWindowManager()
+	var slot *application.WebviewWindow
+	var order []string
+	record := func(label string) windowOp {
+		return func(_ *application.WebviewWindow, created bool) {
+			order = append(order, fmt.Sprintf("%s:%v", label, created))
+		}
+	}
+	var factory func() *application.WebviewWindow
+	factory = func() *application.WebviewWindow {
+		s.withWindow(windowMain, &slot, factory, func(w *application.WebviewWindow, created bool) {
+			record("a")(w, created)
+			// Arrives while the creator is still draining the queue: it must not
+			// jump ahead of "b" through the existing-window fast path.
+			s.withWindow(windowMain, &slot, factory, record("c"))
+		})
+		s.withWindow(windowMain, &slot, factory, record("b"))
+		return &application.WebviewWindow{}
+	}
+
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		s.withWindow(windowMain, &slot, factory, record("outer"))
+	}()
+	waitDone(t, done, "withWindow deadlocked while draining queued operations")
+
+	require.Equal(t, []string{"outer:true", "a:false", "b:false", "c:false"}, order)
+	require.Empty(t, s.creating)
+	require.Empty(t, s.pendingOps)
+}
+
+func TestWithWindowFactoryPanicReleasesCreation(t *testing.T) {
+	s := newTestWindowManager()
+	var slot *application.WebviewWindow
+	func() {
+		defer func() { require.NotNil(t, recover()) }()
+		s.withWindow(windowMain, &slot, func() *application.WebviewWindow {
+			panic("factory failed")
+		}, func(*application.WebviewWindow, bool) {})
+	}()
+	require.Empty(t, s.creating)
+	require.Empty(t, s.pendingOps)
+	require.Nil(t, slot)
+
+	created := false
+	s.withWindow(windowMain, &slot, func() *application.WebviewWindow {
+		return &application.WebviewWindow{}
+	}, func(_ *application.WebviewWindow, c bool) {
+		created = c
+	})
+	require.True(t, created)
+	require.NotNil(t, slot)
+}
+
+func TestWithWindowNilFromFactoryReleasesCreation(t *testing.T) {
+	s := newTestWindowManager()
+	var slot *application.WebviewWindow
+	opCalls := 0
+	s.withWindow(windowMain, &slot, func() *application.WebviewWindow {
+		return nil
+	}, func(*application.WebviewWindow, bool) {
+		opCalls++
+	})
+	require.Equal(t, 0, opCalls)
+	require.Empty(t, s.creating)
+	require.Nil(t, slot)
+}