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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 */
#include "mozilla/Attributes.h"
#include "mozilla/gfx/MatrixFwd.h"
#include "mozilla/webrender/WebRenderAPI.h"
#include "mozilla/webrender/WebRenderTypes.h"
#include "Units.h"
namespace mozilla {
class nsDisplayTransform;
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 {
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.
// Pops the stacking context, if one was pushed during the constructor.
// Export the inherited scale
gfx::MatrixScales GetInheritedScale() const { return mScale; }
const gfx::Matrix& GetInheritedTransform() const {
return mInheritedTransform;
const gfx::Matrix& GetSnappingSurfaceTransform() const {
return mSnappingSurfaceTransform;
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; }
wr::DisplayListBuilder* mBuilder;
gfx::MatrixScales 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 nullptr, 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.
nsDisplayTransform* mDeferredTransformItem;
Maybe<gfx::Matrix4x4> mDeferredAncestorTransform;
bool mRasterizeLocally;
} // namespace layers
} // namespace mozilla