Forcing window to redraw itself?

[cbx <bnucbx gmail com>]

Hi,there
I have been writing a program with the picture as follows:
1, a certain number of Label is packed into a VBox
2, the VBox is put in a Layout
3, the Layout is add to a TOPLEVEL window
4, you can drag the VBox up and down as you wish.

The problem is:when you drag it upwards or downwards,the track of VBox will be left on the screen.
I guess it is because the TOPLEVEL window doesn't redraw it immediately.But I'm not sure of that.

Can you help me by looking into it?
Thank you!

For code(in Python),please see the attachment.

Attachment: example.py
Description: Binary data