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

Steps to recreate:

[LIST=1]
[*]Set drawing mode to pool, auto, or alias
[*]Scroll down in the rightmost combo list a bit
[*]Click a combo in the bottommost visible row of that combo list
[*]Slowly scroll back up to the top of the combo list
[*]Observe selection square goes past bottom of combo list
[/LIST]