util_trayHiddenChange, which means an API change. In board, calculate
bottom of board based on bottom of tray when tray is hidden, and round
so that it's an exact multiple of cell height. In wince, take
advantage of the new parameter to fix the scrollbar so that very
constrained boards can be scrolled even when the tray is hidden.
distinction between obscuring and completely hiding. In former case,
erase the tray rather than draw it partially hidden. This means
drawing the tray before rather than after the board, which is a big
change that so far doesn't seem to have hurt anything. If it does, it
might be necessary to draw it before and after the board.