[wayland] Implement notify in ApplicationWayland

If we get an Expose event for any QWindow we need to ensure that all
wayland events on both client (Qt) and server (KWin) side are dispatched.
Otherwise it could happen that Qt starts rendering although the last
frame callback has not yet been emitted, causing the main gui thread to
be blocked.

main_wayland.cpp

@@ -251,6 +251,19 @@ void ApplicationWayland::createX11Connection()
     setX11RootWindow(defaultScreen()->root);
 }
 
+bool ApplicationWayland::notify(QObject *o, QEvent *e)
+{
+    if (qobject_cast< QWindow* >(o)) {
+        if (e->type() == QEvent::Expose) {
+            // ensure all Wayland events both on client and server side are dispatched
+            // otherwise it is possible that Qt starts rendering and blocks the main gui thread
+            // as it waits for the callback of the last frame
+            waylandServer()->dispatch();
+        }
+    }
+    return Application::notify(o, e);
+}
+
 /**
  * Starts the Xwayland-Server.
  * The new process is started by forking into it.

main_wayland.h

@@ -39,6 +39,8 @@ public:
         m_applicationsToStart = applications;
     }
 
+    bool notify(QObject *o, QEvent *e) override;
+
 protected:
     void performStartup() override;