The images show what I'm building and trying to fix.
1. Normal layout with a tooltip which correctly overlaps the 'Home' dropdown control.
2. Having clicked on the dropdown control, all the buttons that have a tooltip are still shown higher. In this situation the dropdown should be on top.
I've read articles on stacking context, and seen 2 of KP's videos. But I'm still unsure if what I want is even possible, let alone how.