New Menu: Don't suppress scrollbars while animating

[kfranqueiro]

A long time ago, the old "Simple" Menu used to do some height acrobatics when animating open. This led to scrollbars potentially appearing while the menu was animating open, which in turn led to this fix.

Since we redesigned the menu, this is no longer an issue since we no longer animate heights. However, we're currently still suppressing scrollbars, which leads to odd snap-in effects when the menu finishes its opening transitions. We should probably stop suppressing the scrollbar during opening.