Grapic card: GeForce4 Ti 4200, Microsoft Windows driver v5.6.7.3
But graphic cards aren't in charge of managing control extent zones...
The behavior is the same in both screen shots, only example differs.
First screenshot, menu in standard interface:
Move a single pixel to the right and the menuitem unhighlights and the submenu goes away.
Second screenshot, context menu with skin:
Move a single pixel to the left and submenu item highlight disapears, but the submenu stays.
(side note: Buttons on window behind hot track mouse pointer, they shouldn't.)
Delay between move, unhighlight, and submenu disappearance can be set via
TweakUI, Mouse settings (I set it real fast).
Move the mouse faster than that, and no problem.
Move it slower, and presto !
My guess is that there is a gap between menu and submenu control extent zone, although the graphical extent touches.
PS: I used Paint Shop Pro v8 for screenshots, and it happens to have the exact same menu style, but when doing as described in shot#1 the highlight & submenu stay.