Some of my modes use overlays for contextual information.
However, I have a face that I'd like to always be shown despite any overlays on top of it. Is there any way to have such a "super face"?
If you want a whitespace warning face to win, then use an overlay for that face, and give it a higher
priority value than the overlay you want to shadow.
You can use library Highlight to do this. It lets you highlight using text properties or overlays (or both).