a cursory look through the code indicates that there may indeed be a bug in the screen management between writing the bits on a bitmap and the bitmap to the screen. i have different code for osx which is rearranged to avoid creating a separate thread for handling the display, but that code is only for p9p and can't be dropped in drawterm without a bit of work. with that code we can at least be sure that we have no races and, chance is, you won't get a crash with it.
of course, it's still better to rewrite the whole osx gui code directly in Cocoa, which may happen sooner or later :)
