Motivation

With the native popover styles, the overlayClass property defined in OverlayClassMixin won't be useful.

Currently, it can be used for styles on the overlay elements e.g. like this:

vaadin-select-overlay.custom::part(content) {
  color: red;
}

But with the new approach, part styling should be used this way instead;

vaadin-select.custom::part(content) {
  color: red;
}

Proposed solution

Remove the overlayClass property in V25 as a breaking change, document this in the upgrade guide together with the selector changes that are necessary anyway.