Bug 988511 - Allow fixed position items to fully occlude displayport items. r=tn

We don't allow fixed position items to occlude display-ports, but we should if
they fully occlude the scroll-position scroll clamping port of the displayport
frame.

layout/base/nsDisplayList.cpp

@@ -1181,16 +1181,30 @@ nsDisplayList::ComputeVisibilityForSublist(nsDisplayListBuilder* aBuilder,
       // is that the fixed pos content is in a child document, in which it
       // would scroll with the rest of the content.
       bool occlude = true;
+      nsIPresShell* presShell = nullptr;
       if (aDisplayPortFrame && item->IsInFixedPos()) {
         if (item->Frame()->PresContext() == aDisplayPortFrame->PresContext()) {
           occlude = false;
+          presShell = aDisplayPortFrame->PresContext()->PresShell();
         }
       }
 
+      nsRegion opaque = TreatAsOpaque(item, aBuilder);
       if (occlude) {
-        nsRegion opaque = TreatAsOpaque(item, aBuilder);
         // Subtract opaque item from the visible region
         aBuilder->SubtractFromVisibleRegion(aVisibleRegion, opaque);
+      } else if (presShell &&
+                 presShell->IsScrollPositionClampingScrollPortSizeSet() &&
+                 !opaque.IsEmpty()) {
+        // We make an exception if the fixed position item would fully occlude
+        // the scroll position clamping scroll-port. In that case, it's very
+        // unlikely that it will become visible via async scrolling, so we let
+        // it occlude.
+        nsRect scrollClampingScrollPort(nsPoint(0, 0),
+          presShell->GetScrollPositionClampingScrollPortSize());
+        if (opaque.Contains(scrollClampingScrollPort)) {
+          aVisibleRegion->SetEmpty();
+        }
       }
 
       if (aBuilder->NeedToForceTransparentSurfaceForItem(item) ||