I have reproduced bug with disappearing of parts of onscreen keyboard. It is really seems like this is due to some bugs in how tablet's X.Org works with enabled Composite... When composite mode is enabled, some graphic operations are performed in a different way, because X Server maintains backing store pixmaps for every window and tries to reduce number of redraws. For now, i have no idea of how it could be fixed or workarounded May be some ideas will come later