Bug 1022612. Part 25: We don't need to explicitly worry about displayports anymore when computing occlusion. r=mattwoodrow

--HG--
extra : rebase_source : 22926330830072dd520ab41daa37c451bf35601d

layout/base/nsDisplayList.cpp

@@ -1114,39 +1114,9 @@ nsDisplayList::ComputeVisibilityForSublist(nsDisplayListBuilder* aBuilder,
                                 aAllowVisibleRegionExpansion.Intersect(bounds))) {
       anyVisible = true;
 
-      // If we're in a displayport, we need to make sure that fixed position
-      // items do not subtract from the visible region, as async scrolling
-      // may expose these occluded areas.
-      // If the item is fixed pos in the same document as the displayport
-      // then don't let it occlude this list. The only other case possible
-      // 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) {
-        // 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();
-        }
-      }
+      // Subtract opaque item from the visible region
+      aBuilder->SubtractFromVisibleRegion(aVisibleRegion, opaque);
     }
     AppendToBottom(item);
   }