GTK: When using CEF and yield, process CEF pending events.

We should process CEF pending events when yield, it is essential for clipboard
interection to use the same event loop weaved for two literally indenpendent
applications(our own app and CEF).

src/gtk/evtloop.cpp

@@ -26,6 +26,7 @@
 #ifndef WX_PRECOMP
     #include "wx/app.h"
     #include "wx/log.h"
+    #include "wx/event.h"
 #endif // WX_PRECOMP
 
 #include "wx/private/eventloopsourcesmanager.h"
@@ -411,8 +412,17 @@ void wxGUIEventLoop::DoYieldFor(long eventsToProcess)
                 wxgtk_main_do_event(gdk_event_, this);
                 gdk_event_free(gdk_event_);
             }
-            else
+            else {
+                // while no local gdk events pending,
+                // we should schedule one time trigger to allow events for CEF
+                // to be handled, this is essential to unblock us from clipboard
+                // interaction with CEF.
+                // this bogus event will let the DoMessageLoopWork() kick in.
+                wxCommandEvent cef_jobs_trigger{};
+                if (wxTheApp)
+                    wxTheApp->ProcessEvent(dynamic_cast<wxEvent&>(cef_jobs_trigger));
                 break;
+            }
         }
 	}
 	else {