CursorInvoker: check if event.mode is Gdk.CrossingMode.NORMAL to trigger a mouse leave

Like in the WidgetInvoker we check now first if the leave
event has the mode Gdk.CrossingMode.NORMAL, only then we trigger
a mouse leave to popdown the Palette. This stops the Palette
to appear/disappear in a loop.

Signed-off-by: Simon Schampijer <simon@laptop.org>

src/sugar3/graphics/palettewindow.py

@@ -1106,6 +1106,7 @@ class CursorInvoker(Invoker):
         return False
 
     def __leave_notify_event_cb(self, button, event):
+        if event.mode == Gdk.CrossingMode.NORMAL:
             self.notify_mouse_leave()
         return False