Windows not redrawing after upgrade on os x

Hey folks, wondering if anyone can point me in the right direction here.
I have a wx app with multiple windows and sizers going. I built it
originally on os x 10.4 with python 2.5 and wxpython 2.6. After upgradin
to python 2.6 and wxpython 2.8, some of my panes are not redrawing. It
happens like this:

- click button in pane 1 ( pane 2 is supposed to switch views and
redraw )
- pane 2 goes blank ( background colour no foreground color )
- if I resize the whole outer window, pane 2 redraws properly and has
the correct contents

So I guess something changed in the upgrade, but as I have been out of
the loop for wxpy for a while, I thought I'd see if anyone knows off the
top of their head what it might be. I'm using the latest framework build
of python 2.6 and ansi wxpython.

Thanks
Iain