It seems that the notification overlay was only using OverlayStackMixin to participate in the global z-index coordination logic, and for other the other logic in that mixin it was excluded. Since the z-index is not necessary anymore it looks like the mixin can be removed, and the few lines for bring to front can be duplicated. This also allows removing some extra logic from the mixin.

This also fixes a regression introduced by #9773, where opening a notification after another overlay component triggers an error because _deepContains is not defined on the notification container because it does not have OverlayFocusMixin.

There is one side-effect of this, which is that OverlayStackMixin.bringToFront might not do anything in other overlay components if:

• the overlay component is the last one tracked in the attached instances of the mixin
• however the notification container is at the top of the top-layer stack

I can't think of any issues this could cause at this point. Worst case a modeless dialog might show behind a notification when dragged, whereas currently it would move above it.

Yes, I think the change is fine. When we originally added bringToFront() API in vaadin/vaadin-overlay#168, it was explicitly mentioned that notification was supposed to be on top of everything:

Notification uses separate z-index space (on top of everything) & overlay implementation and thus shouldn't be affected by this change.

I was also thinking about modeless dialog behind a notification, but probably it's ok for now.

All of these tests have become obsolete I believe. As an alternative I tried adding some new tests that verify that the native popover API is used properly, maybe something like this can be done for other overlay components as well. Theoretically we could also have ITs to verify the stacking order in the top layer in different opening order or when using bring to front.

I'm ok with removing ITs for now, we can consider adding new tests in a separate PR.