Bug 1436415 - Part 2: Enable nsIFrame::BuildDisplayListForChild() fast-path when using simple event region items r=mattwoodrow

MozReview-Commit-ID: 77rwS0cDUnn

--HG--
extra : rebase_source : 256cedb216e7e737471da3956b57ca0f876ab306

layout/generic/nsFrame.cpp

@@ -3506,11 +3506,12 @@ nsIFrame::BuildDisplayListForChild(nsDisplayListBuilder*   aBuilder,
 
   aBuilder->ClearWillChangeBudget(child);
 
-  const bool doingShortcut =
+  const bool shortcutPossible = aBuilder->IsPaintingToWindow() &&
+    (aBuilder->IsBuildingLayerEventRegions() ||
+     aBuilder->BuildCompositorHitTestInfo());
+
+  const bool doingShortcut = shortcutPossible &&
     (child->GetStateBits() & NS_FRAME_SIMPLE_DISPLAYLIST) &&
-    aBuilder->IsPaintingToWindow() &&
-    // This would be changed by the change of preference.
-    aBuilder->IsBuildingLayerEventRegions() &&
     // Animations may change the value of |HasOpacity()|.
     !(child->GetContent() &&
       child->GetContent()->MayHaveAnimations());
@@ -3792,15 +3793,15 @@ nsIFrame::BuildDisplayListForChild(nsDisplayListBuilder*   aBuilder,
         if (eventRegions) {
           eventRegions->AddFrame(aBuilder, child);
         }
-        if (!awayFromCommonPath &&
-            aBuilder->IsPaintingToWindow() &&
-            !buildingForChild.MaybeAnimatedGeometryRoot()) {
-          // The shortcut is available for the child for next time.
-          child->AddStateBits(NS_FRAME_SIMPLE_DISPLAYLIST);
-        }
       }
     }
 
+    if (!awayFromCommonPath && shortcutPossible &&
+        !differentAGR && !buildingForChild.MaybeAnimatedGeometryRoot()) {
+      // The shortcut is available for the child for next time.
+      child->AddStateBits(NS_FRAME_SIMPLE_DISPLAYLIST);
+    }
+
     if (!pseudoStackingContext) {
       // THIS IS THE COMMON CASE.
       // Not a pseudo or real stacking context. Do the simple thing and