[zeditor] White combo select square scrolls past bottom of combo lists

Steps to recreate:

1. Set drawing mode to pool, auto, or alias
2. Scroll down in the rightmost combo list a bit
3. Click a combo in the bottommost visible row of that combo list
4. Slowly scroll back up to the top of the combo list
5. Observe selection square goes past bottom of combo list

In Pool drawing mode, the square is drawn by the call to safe_rect right before the //Handle Preview comment