Bug 1918762 - Use ElementState rather than the hover attribute to determine scrollbar hover state. r=mac-reviewers,mstange

That's what we use for painting in all platforms except macOS with
overlay scrollbars, where we make it sticky. Make the stickiness more
explicit (this also fixes the sticky state getting cleaned up too
early).

Differential Revision: https://phabricator.services.mozilla.com/D222161

layout/generic/ScrollContainerFrame.cpp

@@ -3409,7 +3409,8 @@ static void AppendToTop(nsDisplayListBuilder* aBuilder,
 struct HoveredStateComparator {
   static bool Hovered(const nsIFrame* aFrame) {
     return aFrame->GetContent()->IsElement() &&
-           aFrame->GetContent()->AsElement()->HasAttr(nsGkAtoms::hover);
+           aFrame->GetContent()->AsElement()->State().HasState(
+               ElementState::HOVER);
   }
 
   bool Equals(nsIFrame* A, nsIFrame* B) const {