Similar issues:

• egui menus: remember size from previous frame #3974
• ComboBox menu doesn't report its actual size to its closure #4452

Tooltips sometimes flicker the first time they show up.
This is because we don't know their size before we show them, and their size affects their position.

I think we should hide the tooltips for the first frame, and use that only for sizing calculations.
Once calculated, we store the size for use the subsequent frames.
The tooltips size will be assumed to be constant during its lifetime.
Once a tooltip is hidden, we forget about its size.

This will also allow for better sizing within a tooltip: a user will be able to add "full width" widgets (such as Separator) that are only as wide as the widest element.