Bug 1528743 - Move the top-layer display items inside the async zoom container. r=mstange

This maintains the important invariant that layers that carry scroll metadata
for the RCD-RSF are inside the async zoom container.

Differential Revision: https://phabricator.services.mozilla.com/D22049

--HG--
extra : moz-landing-system : lando

layout/generic/ViewportFrame.cpp

@@ -60,18 +60,6 @@ void ViewportFrame::BuildDisplayList(nsDisplayListBuilder* aBuilder,
     // below negative z-index elements.
     BuildDisplayListForChild(aBuilder, kid, aLists);
   }
-
-  nsDisplayList topLayerList;
-  BuildDisplayListForTopLayer(aBuilder, &topLayerList);
-  if (!topLayerList.IsEmpty()) {
-    // Wrap the whole top layer in a single item with maximum z-index,
-    // and append it at the very end, so that it stays at the topmost.
-    nsDisplayWrapList* wrapList =
-        MakeDisplayItem<nsDisplayWrapList>(aBuilder, this, &topLayerList);
-    wrapList->SetOverrideZIndex(
-        std::numeric_limits<decltype(wrapList->ZIndex())>::max());
-    aLists.PositionedDescendants()->AppendToTop(wrapList);
-  }
 }
 
 #ifdef DEBUG

layout/generic/nsGfxScrollFrame.cpp

@@ -68,6 +68,7 @@
 #include "UnitTransforms.h"
 #include "nsPluginFrame.h"
 #include "nsSliderFrame.h"
+#include "ViewportFrame.h"
 #include "mozilla/layers/APZCCallbackHelper.h"
 #include "mozilla/layers/AxisPhysicsModel.h"
 #include "mozilla/layers/AxisPhysicsMSDModel.h"
@@ -3378,6 +3379,8 @@ void ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder* aBuilder,
       mOuter->BuildDisplayListForChild(aBuilder, mScrolledFrame, aLists);
     }
 
+    MaybeAddTopLayerItems(aBuilder, aLists);
+
     if (addScrollBars) {
       // Add overlay scrollbars.
       AppendScrollPartsTo(aBuilder, aLists, createLayersForScrollbars, true);
@@ -3662,6 +3665,8 @@ void ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder* aBuilder,
     }
   }
 
+  MaybeAddTopLayerItems(aBuilder, set);
+
   if (willBuildAsyncZoomContainer) {
     MOZ_ASSERT(mClipAllDescendants);
 
@@ -3735,6 +3740,25 @@ void ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder* aBuilder,
   scrolledContent.MoveTo(aLists);
 }
 
+void ScrollFrameHelper::MaybeAddTopLayerItems(nsDisplayListBuilder* aBuilder,
+                                              const nsDisplayListSet& aLists) {
+  if (mIsRoot) {
+    if (ViewportFrame* viewportFrame = do_QueryFrame(mOuter->GetParent())) {
+      nsDisplayList topLayerList;
+      viewportFrame->BuildDisplayListForTopLayer(aBuilder, &topLayerList);
+      if (!topLayerList.IsEmpty()) {
+        // Wrap the whole top layer in a single item with maximum z-index,
+        // and append it at the very end, so that it stays at the topmost.
+        nsDisplayWrapList* wrapList = MakeDisplayItem<nsDisplayWrapList>(
+            aBuilder, viewportFrame, &topLayerList);
+        wrapList->SetOverrideZIndex(
+            std::numeric_limits<decltype(wrapList->ZIndex())>::max());
+        aLists.PositionedDescendants()->AppendToTop(wrapList);
+      }
+    }
+  }
+}
+
 bool ScrollFrameHelper::DecideScrollableLayer(
     nsDisplayListBuilder* aBuilder, nsRect* aVisibleRect, nsRect* aDirtyRect,
     bool aSetBase, bool* aDirtyRectHasBeenOverriden) {

layout/generic/nsGfxScrollFrame.h

@@ -82,6 +82,18 @@ class ScrollFrameHelper : public nsIReflowCallback {
   void BuildDisplayList(nsDisplayListBuilder* aBuilder,
                         const nsDisplayListSet& aLists);
 
+  // Add display items for the top-layer (which includes things like
+  // the fullscreen element, its backdrop, and text selection carets)
+  // to |aLists|.
+  // This is a no-op for scroll frames other than the viewport's
+  // root scroll frame.
+  // This should be called with an nsDisplayListSet that will be
+  // wrapped in the async zoom container, if we're building one.
+  // It should not be called with an ASR setter on the stack, as the
+  // top-layer items handle setting up their own ASRs.
+  void MaybeAddTopLayerItems(nsDisplayListBuilder* aBuilder,
+                             const nsDisplayListSet& aLists);
+
   void AppendScrollPartsTo(nsDisplayListBuilder* aBuilder,
                            const nsDisplayListSet& aLists, bool aCreateLayer,
                            bool aPositioned);