1 files changed, 9 insertions, 20 deletions

diff --git a/gfx/layers/wr/WebRenderCommandBuilder.cpp b/gfx/layers/wr/WebRenderCommandBuilder.cpp
index e1bb2e1127..d7c7468f56 100644
--- a/gfx/layers/wr/WebRenderCommandBuilder.cpp
+++ b/gfx/layers/wr/WebRenderCommandBuilder.cpp
@@ -1882,9 +1882,8 @@ struct NewLayerData {
   ScrollableLayerGuid::ViewID mDeferredId = ScrollableLayerGuid::NULL_SCROLL_ID;
   bool mTransformShouldGetOwnLayer = false;
 
-  void ComputeDeferredTransformInfo(
-      const StackingContextHelper& aSc, nsDisplayItem* aItem,
-      nsDisplayTransform* aLastDeferredTransform) {
+  void ComputeDeferredTransformInfo(const StackingContextHelper& aSc,
+                                    nsDisplayItem* aItem) {
     // See the comments on StackingContextHelper::mDeferredTransformItem
     // for an overview of what deferred transforms are.
     // In the case where we deferred a transform, but have a child display
@@ -1900,14 +1899,6 @@ struct NewLayerData {
     // that we deferred, and a child WebRenderLayerScrollData item that
     // holds the scroll metadata for the child's ASR.
     mDeferredItem = aSc.GetDeferredTransformItem();
-    // If this deferred transform is already slated to be emitted onto an
-    // ancestor layer, do not emit it on this layer as well. Note that it's
-    // sufficient to check the most recently deferred item here, because
-    // there's only one per stacking context, and we emit it when changing
-    // stacking contexts.
-    if (mDeferredItem == aLastDeferredTransform) {
-      mDeferredItem = nullptr;
-    }
     if (mDeferredItem) {
       // It's possible the transform's ASR is not only an ancestor of
       // the item's ASR, but an ancestor of stopAtAsr. In such cases,
@@ -2071,10 +2062,7 @@ void WebRenderCommandBuilder::CreateWebRenderCommandsFromDisplayList(
         newLayerData->mLayerCountBeforeRecursing = mLayerScrollData.size();
         newLayerData->mStopAtAsr =
             mAsrStack.empty() ? nullptr : mAsrStack.back();
-        newLayerData->ComputeDeferredTransformInfo(
-            aSc, item,
-            mDeferredTransformStack.empty() ? nullptr
-                                            : mDeferredTransformStack.back());
+        newLayerData->ComputeDeferredTransformInfo(aSc, item);
 
         // Ensure our children's |stopAtAsr| is not be an ancestor of our
         // |stopAtAsr|, otherwise we could get cyclic scroll metadata
@@ -2096,10 +2084,12 @@ void WebRenderCommandBuilder::CreateWebRenderCommandsFromDisplayList(
         mAsrStack.push_back(stopAtAsrForChildren);
 
         // If we're going to emit a deferred transform onto this layer,
-        // keep track of that so descendant layers know not to emit the
-        // same deferred transform.
+        // clear the deferred transform from the StackingContextHelper
+        // while we are building the subtree of descendant layers.
+        // This ensures that the deferred transform is not applied in
+        // duplicate to any of our descendant layers.
         if (newLayerData->mDeferredItem) {
-          mDeferredTransformStack.push_back(newLayerData->mDeferredItem);
+          aSc.ClearDeferredTransformItem();
         }
       }
     }
@@ -2143,8 +2133,7 @@ void WebRenderCommandBuilder::CreateWebRenderCommandsFromDisplayList(
         mAsrStack.pop_back();
 
         if (newLayerData->mDeferredItem) {
-          MOZ_ASSERT(!mDeferredTransformStack.empty());
-          mDeferredTransformStack.pop_back();
+          aSc.RestoreDeferredTransformItem(newLayerData->mDeferredItem);
         }
 
         const ActiveScrolledRoot* stopAtAsr = newLayerData->mStopAtAsr;