Bug 1276107 - Ensure sorting display items by z-order doesn't cause event-regions items to end up below items they are supposed to cover. r=mattwoodrow

MozReview-Commit-ID: BxnshG9TgRb

--HG--
extra : rebase_source : 5817e50d64a1211e224c9b6dbd3e3efdcd5e48f0
extra : intermediate-source : 6f03149a7565e317156cc7218120f39c9fb41915
extra : source : 4c1ca58448e3513691f92c1f9f0a92329ccc17c5
extra : histedit_source : d3ddaa7425ac1e2a1b019c957525653aeee8a4da

layout/base/nsDisplayList.cpp

@@ -3695,6 +3695,18 @@ nsDisplayLayerEventRegions::IsEmpty() const
   return false;
 }
 
+int32_t
+nsDisplayLayerEventRegions::ZIndex() const
+{
+  return mOverrideZIndex ? *mOverrideZIndex : nsDisplayItem::ZIndex();
+}
+
+void
+nsDisplayLayerEventRegions::SetOverrideZIndex(int32_t aZIndex)
+{
+  mOverrideZIndex = Some(aZIndex);
+}
+
 void
 nsDisplayLayerEventRegions::WriteDebugInfo(std::stringstream& aStream)
 {

layout/base/nsDisplayList.h

@@ -16,6 +16,7 @@
 #include "mozilla/Attributes.h"
 #include "mozilla/DebugOnly.h"
 #include "mozilla/EnumSet.h"
+#include "mozilla/Maybe.h"
 #include "nsCOMPtr.h"
 #include "nsContainerFrame.h"
 #include "nsPoint.h"
@@ -3141,6 +3142,10 @@ public:
   void AddInactiveScrollPort(const nsRect& aRect);
 
   bool IsEmpty() const;
+
+  int32_t ZIndex() const override;
+  void SetOverrideZIndex(int32_t aZIndex);
+
   const nsRegion& HitRegion() { return mHitRegion; }
   const nsRegion& MaybeHitRegion() { return mMaybeHitRegion; }
   const nsRegion& DispatchToContentHitRegion() { return mDispatchToContentHitRegion; }
@@ -3169,6 +3174,11 @@ private:
   // These are points where panning is vertical, as determined by the touch-action
   // property. Always contained in the union of mHitRegion and mMaybeHitRegion.
   nsRegion mVerticalPanRegion;
+  // If these event regions are for an inactive scroll frame, the z-index of
+  // this display item is overridden to be the largest z-index of the content
+  // in the scroll frame. This ensures that the event regions item remains on
+  // top of the content after sorting items by z-index.
+  mozilla::Maybe<int32_t> mOverrideZIndex;
 };
 
 /**

layout/generic/nsGfxScrollFrame.cpp

@@ -3463,6 +3463,7 @@ ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
         MaxZIndexInListOfItemsContainedInFrame(positionedDescendants, mOuter);
       if (zindex >= 0) {
         destinationList = positionedDescendants;
+        inactiveRegionItem->SetOverrideZIndex(zindex);
       } else {
         destinationList = scrolledContent.Outlines();
       }