When you click to open a stack, and then you immediately click again to close it, nothing happens. The mouse must be moved a tad in order for the next click to register. I'm not sure if this is a "feature" to protect against accidental double clicks closing a stack right after it's opening, but if it is, it's a lousy feature.
Also, after closing the stack, clicking again (with the desire to reopen the stack) does nothing. The mouse must be moved a tad before clicking again in order to get the stack to reopen. No matter how many times you click after the stack has closed, if the mouse hasn't moved, those clicks will do nothing.
I've run into this many times, either clicking the wrong stack and wanting to close it right after opening it, or wanting to reopen a stack right after closing it for whatever reason, and it drives me nuts.
It seems like a bug/glitch, should be an easy fix. If it's a feature, get rid of it.