Fixed moving window crash in Windows.

tests/testapp.py

@@ -95,7 +95,7 @@ def create_context_window(w, h, callback):
         root.set_position(*event.get_coords())
         root.set_size(event.width, event.height)
         overlay.set_size(event.width, event.height)
-        draw()
+        refresh()
 
     def handle_key(win, event):
         """Handle key event. 'f' toggles fullscreen, 'q' exits the program, 'b'