After change T158934: Automatically change popup direction if there is no space, OOUI menus and popups can open upwards instead of downwards. This was meant to make long dropdowns easier to use, see for example T107036#3796820.
I didn't foresee that various interface elements shown near the top of the page can overlap these menus/popups. This task is about the general solution for this problem, and also tracks all the specific instances that were reported, so that we can verify they're all fixed.