Works here (debian 64-bit, wxWidgets 2.8.10, xfce4, radeonhd graphics-driver) with composite effects on and off.
Does this happen for docked, unocked or both windows ?
Are all other windows and panes shown correctly ?
If it happens for undocked windows, what happens if you undock other panes (Management, Logs and others or toolbars) ?
Did you try to play with the composite settings ?
When did it stop working ?
We did not change related code in the last revisions.
So it might be related to other updates on your system especially for gtk+, xorg or your graphics-driver.