The panels of main frame are resized strange

[blackmonkey]

Reproduce steps

1. launch application
2. drag main frame right/left border back and force
3. observe:
   1. the vertical splitter trembles
   2. if the main frame is resized too narrow, the vertical splitter keeps moving left and reach the left border.
4. drag main frame top/bottom border back and force
5. observe: the vertical splitter trembles as previous, while the horizontal splitter keeps moving up and reach the top border.