mus: Fix handled status in UI event ack, add MessageLoop::NestingObserver

ui/views/mus/platform_window_mus.h

Index: ui/views/mus/platform_window_mus.h
diff --git a/ui/views/mus/platform_window_mus.h b/ui/views/mus/platform_window_mus.h
index 9364239879998bc6493194450c2b303e73a33d68..d0e59d9f9a64344370e12f5d6c324a3f99c5fd39 100644
--- a/ui/views/mus/platform_window_mus.h
+++ b/ui/views/mus/platform_window_mus.h
@@ -11,6 +11,7 @@
 #include <vector>
 
 #include "base/macros.h"
+#include "base/message_loop/message_loop.h"
 #include "components/mus/public/cpp/input_event_handler.h"
 #include "components/mus/public/cpp/window_observer.h"
 #include "ui/platform_window/platform_window.h"
@@ -34,7 +35,8 @@ namespace views {
 class VIEWS_MUS_EXPORT PlatformWindowMus
     : public NON_EXPORTED_BASE(ui::PlatformWindow),
       public mus::WindowObserver,
-      public NON_EXPORTED_BASE(mus::InputEventHandler) {
+      public NON_EXPORTED_BASE(mus::InputEventHandler),
+      public base::MessageLoop::NestingObserver {
  public:
   PlatformWindowMus(ui::PlatformWindowDelegate* delegate,
                     shell::Connector* connector,
@@ -89,11 +91,17 @@ class VIEWS_MUS_EXPORT PlatformWindowMus
       std::unique_ptr<base::Callback<void(mus::mojom::EventResult)>>*
           ack_callback) override;
 
+  // base::MessageLoop::NestingObserver:
+  void OnBeginNestedMessageLoop() override;
+
   ui::PlatformWindowDelegate* delegate_;
   mus::Window* mus_window_;
   mus::mojom::ShowState show_state_;
   mus::mojom::Cursor last_cursor_;
 
+  std::unique_ptr<base::Callback<void(mus::mojom::EventResult)>>
+      event_ack_callback_;
+
   // True if OnWindowDestroyed() has been received.
   bool mus_window_destroyed_;