Have the active category's subs expanded in the sidebox instead of the hoverbox? Could be a good idea, especially if the current subcat is not prominently identified in the main column.

Add to the bottom of your stylesheet

.cat-parent-text+ul {display: block !important;}

It needs some more rule tweaking to eliminate a hover jitter, but that would be a start.
I have to go now, and probably won't be able to spend time on this for a couple of days.