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diff --git a/gfx/layers/wr/StackingContextHelper.h b/gfx/layers/wr/StackingContextHelper.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef GFX_STACKINGCONTEXTHELPER_H
+#define GFX_STACKINGCONTEXTHELPER_H
+
+#include "mozilla/Attributes.h"
+#include "mozilla/gfx/MatrixFwd.h"
+#include "mozilla/webrender/WebRenderAPI.h"
+#include "mozilla/webrender/WebRenderTypes.h"
+#include "Units.h"
+
+class nsDisplayTransform;
+
+namespace mozilla {
+
+struct ActiveScrolledRoot;
+
+namespace layers {
+
+/**
+ * This is a helper class that pushes/pops a stacking context, and manages
+ * some of the coordinate space transformations needed.
+ */
+class MOZ_RAII StackingContextHelper {
+ public:
+  StackingContextHelper(const StackingContextHelper& aParentSC,
+                        const ActiveScrolledRoot* aAsr,
+                        nsIFrame* aContainerFrame,
+                        nsDisplayItem* aContainerItem,
+                        wr::DisplayListBuilder& aBuilder,
+                        const wr::StackingContextParams& aParams,
+                        const LayoutDeviceRect& aBounds = LayoutDeviceRect());
+
+  // This version of the constructor should only be used at the root level
+  // of the tree, so that we have a StackingContextHelper to pass down into
+  // the RenderLayer traversal, but don't actually want it to push a stacking
+  // context on the display list builder.
+  StackingContextHelper();
+
+  // Pops the stacking context, if one was pushed during the constructor.
+  ~StackingContextHelper();
+
+  // Export the inherited scale
+  gfx::Size GetInheritedScale() const { return mScale; }
+
+  const gfx::Matrix& GetInheritedTransform() const {
+    return mInheritedTransform;
+  }
+
+  const gfx::Matrix& GetSnappingSurfaceTransform() const {
+    return mSnappingSurfaceTransform;
+  }
+
+  const Maybe<nsDisplayTransform*>& GetDeferredTransformItem() const;
+  Maybe<gfx::Matrix4x4> GetDeferredTransformMatrix() const;
+
+  bool AffectsClipPositioning() const { return mAffectsClipPositioning; }
+  Maybe<wr::WrSpatialId> ReferenceFrameId() const { return mReferenceFrameId; }
+
+  const LayoutDevicePoint& GetOrigin() const { return mOrigin; }
+
+ private:
+  wr::DisplayListBuilder* mBuilder;
+  gfx::Size mScale;
+  gfx::Matrix mInheritedTransform;
+  LayoutDevicePoint mOrigin;
+
+  // The "snapping surface" defines the space that we want to snap in.
+  // You can think of it as the nearest physical surface.
+  // Animated transforms create a new snapping surface, so that changes to their
+  // transform don't affect the snapping of their contents. Non-animated
+  // transforms do *not* create a new snapping surface, so that for example the
+  // existence of a non-animated identity transform does not affect snapping.
+  gfx::Matrix mSnappingSurfaceTransform;
+  bool mAffectsClipPositioning;
+  Maybe<wr::WrSpatialId> mReferenceFrameId;
+  Maybe<wr::SpaceAndClipChainHelper> mSpaceAndClipChainHelper;
+
+  // The deferred transform item is used when building the WebRenderScrollData
+  // structure. The backstory is that APZ needs to know about transforms that
+  // apply to the different APZC instances. Prior to bug 1423370, we would do
+  // this by creating a new WebRenderLayerScrollData for each nsDisplayTransform
+  // item we encountered. However, this was unnecessarily expensive because it
+  // turned out a lot of nsDisplayTransform items didn't have new ASRs defined
+  // as descendants, so we'd create the WebRenderLayerScrollData and send it
+  // over to APZ even though the transform information was not needed in that
+  // case.
+  //
+  // In bug 1423370 and friends, this was optimized by "deferring" a
+  // nsDisplayTransform item when we encountered it during display list
+  // traversal. If we found a descendant of that transform item that had a
+  // new ASR or otherwise was "relevant to APZ", we would then pluck the
+  // transform matrix off the deferred item and put it on the
+  // WebRenderLayerScrollData instance created for that APZ-relevant descendant.
+  //
+  // One complication with this is if there are multiple nsDisplayTransform
+  // items in the ancestor chain for the APZ-relevant item. As we traverse the
+  // display list, we will defer the outermost nsDisplayTransform item, and when
+  // we encounter the next one we will need to merge it with the already-
+  // deferred one somehow. What we do in this case is have
+  // mDeferredTransformItem always point to the "innermost" deferred transform
+  // item (i.e. the closest ancestor nsDisplayTransform item of the item that
+  // created this StackingContextHelper). And then we use
+  // mDeferredAncestorTransform to store the product of all the other transforms
+  // that were deferred. As a result, there is an invariant here that if
+  // mDeferredTransformItem is Nothing(), mDeferredAncestorTransform will also
+  // be Nothing(). Note that we can only do this if the nsDisplayTransform items
+  // share the same ASR. If we are processing an nsDisplayTransform item with a
+  // different ASR than the previously-deferred item, we assume that the
+  // previously-deferred transform will get sent to APZ as part of a separate
+  // WebRenderLayerScrollData item, and so we don't need to bother with any
+  // merging. (The merging probably wouldn't even make sense because the
+  // coordinate spaces might be different in the face of async scrolling). This
+  // behaviour of forcing a WebRenderLayerScrollData item to be generated when
+  // the ASR changes is implemented in
+  // WebRenderCommandBuilder::CreateWebRenderCommandsFromDisplayList.
+  Maybe<nsDisplayTransform*> mDeferredTransformItem;
+  Maybe<gfx::Matrix4x4> mDeferredAncestorTransform;
+
+  bool mRasterizeLocally;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif /* GFX_STACKINGCONTEXTHELPER_H */