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diff --git a/gfx/layers/wr/ClipManager.h b/gfx/layers/wr/ClipManager.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_CLIPMANAGER_H
+#define GFX_CLIPMANAGER_H
+
+#include <stack>
+#include <unordered_map>
+
+#include "mozilla/Attributes.h"
+#include "mozilla/webrender/WebRenderAPI.h"
+
+namespace mozilla {
+
+class nsDisplayItem;
+struct ActiveScrolledRoot;
+struct DisplayItemClipChain;
+
+namespace wr {
+class DisplayListBuilder;
+}
+
+namespace layers {
+
+class StackingContextHelper;
+class WebRenderLayerManager;
+
+/**
+ * This class manages creating and assigning scroll layers and clips in
+ * WebRender based on the gecko display list. It has a few public functions that
+ * are intended to be invoked while traversing the Gecko display list, and it
+ * uses the ASR and clip information from the display list to create the
+ * necessary clip state in WebRender.
+ *
+ * The structure of the clip state in WebRender ends up quite similar to how
+ * it is in Gecko. For each ASR in Gecko, we create a scroll layer (i.e. a
+ * scrolling clip) in WebRender; these form a tree structure similar to the
+ * ASR tree structure. Ancestors of scroll layers are always other scroll
+ * layers, or the root scroll node.
+ * The DisplayItemClipChain list of clips from the gecko display list is
+ * converted to a WR clip chain and pushed on the stack prior to creating
+ * any WR commands for that item, and is popped afterwards. In addition,
+ * the WR clip chain has a parent pointer, which points to the clip chain for
+ * any enclosing stacking context. This again results in a strucuture very
+ * similar to that in Gecko, where the clips from container display items get
+ * applied to the contained display items.
+ */
+class ClipManager {
+ public:
+  ClipManager();
+
+  void BeginBuild(WebRenderLayerManager* aManager,
+                  wr::DisplayListBuilder& aBuilder);
+  void EndBuild();
+
+  void BeginList(const StackingContextHelper& aStackingContext);
+  void EndList(const StackingContextHelper& aStackingContext);
+
+  wr::WrSpaceAndClipChain SwitchItem(nsDisplayListBuilder* aBuilder,
+                                     nsDisplayItem* aItem);
+  ~ClipManager();
+
+  void PushOverrideForASR(const ActiveScrolledRoot* aASR,
+                          const wr::WrSpatialId& aSpatialId);
+  void PopOverrideForASR(const ActiveScrolledRoot* aASR);
+
+ private:
+  wr::WrSpatialId SpatialIdAfterOverride(const wr::WrSpatialId& aSpatialId);
+  wr::WrSpatialId GetScrollLayer(const ActiveScrolledRoot* aASR);
+
+  Maybe<wr::WrSpatialId> DefineScrollLayers(const ActiveScrolledRoot* aASR,
+                                            nsDisplayItem* aItem);
+
+  Maybe<wr::WrClipChainId> DefineClipChain(const DisplayItemClipChain* aChain,
+                                           int32_t aAppUnitsPerDevPixel);
+
+  WebRenderLayerManager* MOZ_NON_OWNING_REF mManager;
+  wr::DisplayListBuilder* mBuilder;
+
+  // Stack of clip caches. Each cache contains a map from gecko
+  // DisplayItemClipChain objects to webrender WrClipIds, which allows us to
+  // avoid redefining identical clips in WR. However, the gecko
+  // DisplayItemClipChain items get deduplicated quite aggressively, without
+  // regard to things like the enclosing reference frame or mask. On the WR
+  // side, we cannot deduplicate clips that aggressively. So what we do is
+  // any time we enter a new reference frame (for example) we create a new clip
+  // cache on mCacheStack. This ensures we continue caching stuff within a given
+  // reference frame, but disallow caching stuff across reference frames. In
+  // general we need to do this anytime PushOverrideForASR is called, as that is
+  // called for the same set of conditions for which we cannot deduplicate
+  // clips.
+  using ClipIdMap = std::unordered_map<const DisplayItemClipChain*,
+                                       AutoTArray<wr::WrClipId, 4>>;
+  std::stack<ClipIdMap> mCacheStack;
+
+  // A map that holds the cache overrides created by (a) "out of band" clips,
+  // i.e. clips that are generated by display items but that ClipManager
+  // doesn't know about and (b) stacking contexts that affect clip positioning.
+  // These are called "cache overrides" because while we're inside these things,
+  // we cannot use the ASR from the gecko display list as-is. Fundamentally this
+  // results from a mismatch between the ASR+clip items on the gecko side and
+  // the ClipScrollTree on the WR side; the WR side incorporates things like
+  // transforms and stacking context origins while the gecko side manages those
+  // differently.
+  // Any time ClipManager wants to define a new clip as a child of ASR X, it
+  // should first check the cache overrides to see if there is a cache override
+  // item ((a) or (b) above) that is already a child of X, and then define that
+  // clip as a child of Y instead. This map stores X -> Y, which allows
+  // ClipManager to do the necessary lookup. Note that there theoretically might
+  // be multiple different "Y" clips (in case of nested cache overrides), which
+  // is why we need a stack.
+  std::unordered_map<wr::WrSpatialId, std::stack<wr::WrSpatialId>> mASROverride;
+
+  // This holds some clip state for a single nsDisplayItem
+  struct ItemClips {
+    ItemClips(const ActiveScrolledRoot* aASR,
+              const DisplayItemClipChain* aChain, int32_t aAppUnitsPerDevPixel,
+              bool aSeparateLeaf);
+
+    // These are the "inputs" - they come from the nsDisplayItem
+    const ActiveScrolledRoot* mASR;
+    const DisplayItemClipChain* mChain;
+    int32_t mAppUnitsPerDevPixel;
+    bool mSeparateLeaf;
+
+    // These are the "outputs" - they are pushed to WR as needed
+    wr::WrSpatialId mScrollId;
+    Maybe<wr::WrClipChainId> mClipChainId;
+
+    void UpdateSeparateLeaf(wr::DisplayListBuilder& aBuilder,
+                            int32_t aAppUnitsPerDevPixel);
+    bool HasSameInputs(const ItemClips& aOther);
+    wr::WrSpaceAndClipChain GetSpaceAndClipChain() const;
+  };
+
+  // A stack of ItemClips corresponding to the nsDisplayItem ancestry. Each
+  // time we recurse into a nsDisplayItem's child list, this stack size
+  // increases by one. The topmost item on the stack is for the display item
+  // we are currently processing and items deeper on the stack are for that
+  // display item's ancestors.
+  std::stack<ItemClips> mItemClipStack;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif