PocketMatrix

Where have you seen the "don't rely on previous contents"? The only thing I'm aware of is the Lock() function in PF that has a flag to discard contents on unlock.

Are you using pocketfrog or phal (or both?)

I saw the same sort of thing when I tried Space Treker on my old iPaq h3650. The screen was fine on my Axim but on the iPaq stuff I did not re-draw was missing. You have to redraw the whole screen every frame I guess to ensure that it's always there.