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That's how it should be! You have to give a way to the user change it's mind before doing anything.
If you change your mind after opening a menu you close it. I don't see your point here.
...and we should not forget too that middle click usually does some other default action in most of applications (looks firefox and page scrooling and tab closure with middle click...), or are used for fast document scrolling (also, firefox for example)
And right click doesn't do anything in apps?
Just fyi, middle click pastes the last selection in X. It's still not a problem, because one is a click and the other is a drag.