Make `Memory::keep_popup_open` less of a footgun

[lucasmerlin]

#5814 made it so popups would close unless you keep calling keep_popup_open each frame. In theory this is a nice improvement but it's caused some bugs since and I'm certain more will happen if we release this in the current state. I suggest we find a better solution, or at least make it a (non-silent) breaking change so people are forced to think about it.