missoctavia, I use five seperate single tabs with both XP and Vista and they allow mouseover activation on top of open windows as illustrated below.
I don't know if you're using single or multiple tabs, or even if this makes a difference to your particular configuration, but single tabs does work and act this way for me.
No different or particular settings, I've always found this to be a bit annoying every time I touch a corner, especially the upper right where your close windows button is located, but it's a nice feature to have when you need it.

FYI: This is from my XP box.
Cp