I noticed that Qt programs were less likely to use sub-menus, probably for this reason.

It's doing something; doing "nothing" is what Bootstrap does. But Qt 4 actually seems to go out of it's way to be unpredictable.

     _____
    |item1|_______
    |item2|item2.1|
    |item3|item2.2|
    |item4|

If I drag my cursor from item2 to item2.2 through item3 qucikly, there's clearly a delay, as I see item2.2 get highlighted, but then the submenu disappears, and it selects item3. However, if I overshoot and drag the cursor out of the menu altogether, it leaves item2.2 selected.

But then I think it does something with the angle it is moving at to make that sometimes not work.

The behaviour is complex and unpredictable, which is different than nothing.