I move the cursor slowly from above downwards to the button untill the other buttons are poping up.
After moving the cursor away without highlighting the source button (edit, this doesn't matter: "or one of the poped up buttons")
the poped up buttons won't collaps.
There seems to be an area wich activates popup but will not register the mouse is over the button.
I hope my english is passably understandable.
(already had this effect before and resizing buttons helped me out, but i made a fresh interface yesterday and i like the size of my buttons ^^)