r=pedemont, no sr.
OK, I need this in. OS/2 menus are unusable without this.
Basically, when 117730 went in, it regressed this because
the fix for 101771 was no longer needed.
This is backing out the 101771 fix.
I have verified this doesn't break Windows.
The only other platform that could be an issue is Mac, but
it shouldn't be because Mac uses native menus. No other
platform handles clipSiblings.
If this does break something, you would see it as menus
appearing behind other menus.
