Bug 1449478 - Avoid clobbering transforms in the WebRenderScrollData. r=jrmuizel

Due to an oversight in bug 1423370, the code I added was setting the
transform on a WebRenderLayerScrollData after initializing it, but the
initialization might have populated the transform. Thus the
transform-set that I added would have clobbered the transform. This
updates the code to combine the two transforms instead which avoids
the clobber.

MozReview-Commit-ID: 4mKJTLSMD9J

--HG--
extra : rebase_source : c486c5866739ab040d81f9f9a43b2b8a04c2d383

gfx/layers/wr/StackingContextHelper.cpp

@@ -74,10 +74,10 @@ StackingContextHelper::ToRelativeLayoutRect(const LayoutDeviceRect& aRect) const
   return wr::ToLayoutRect(rect);
 }
 
-gfx::Matrix4x4
+const Maybe<gfx::Matrix4x4>&
 StackingContextHelper::GetTransformForScrollData() const
 {
-  return mTransformForScrollData.valueOr(gfx::Matrix4x4());
+  return mTransformForScrollData;
 }
 
 } // namespace layers

gfx/layers/wr/StackingContextHelper.h

@@ -75,7 +75,7 @@ public:
     return mInheritedTransform;
   }
 
-  gfx::Matrix4x4 GetTransformForScrollData() const;
+  const Maybe<gfx::Matrix4x4>& GetTransformForScrollData() const;
 
   bool AffectsClipPositioning() const { return mAffectsClipPositioning; }
 

gfx/layers/wr/WebRenderCommandBuilder.cpp

@@ -1313,8 +1313,8 @@ WebRenderCommandBuilder::CreateWebRenderCommandsFromDisplayList(nsDisplayList* a
         int32_t descendants = mLayerScrollData.size() - layerCountBeforeRecursing;
 
         mLayerScrollData.emplace_back();
-        mLayerScrollData.back().Initialize(mManager->GetScrollData(), item, descendants, stopAtAsr);
-        mLayerScrollData.back().SetTransform(aSc.GetTransformForScrollData());
+        mLayerScrollData.back().Initialize(mManager->GetScrollData(), item,
+            descendants, stopAtAsr, aSc.GetTransformForScrollData());
       } else if (mLayerScrollData.size() != layerCountBeforeRecursing &&
                  !eventRegions.IsEmpty()) {
         // We are not forcing a new layer for |item|, but we did create some

gfx/layers/wr/WebRenderScrollData.cpp

@@ -42,7 +42,8 @@ void
 WebRenderLayerScrollData::Initialize(WebRenderScrollData& aOwner,
                                      nsDisplayItem* aItem,
                                      int32_t aDescendantCount,
-                                     const ActiveScrolledRoot* aStopAtAsr)
+                                     const ActiveScrolledRoot* aStopAtAsr,
+                                     const Maybe<gfx::Matrix4x4>& aTransform)
 {
   MOZ_ASSERT(aDescendantCount >= 0); // Ensure value is valid
   MOZ_ASSERT(mDescendantCount == -1); // Don't allow re-setting an already set value
@@ -50,6 +51,11 @@ WebRenderLayerScrollData::Initialize(WebRenderScrollData& aOwner,
 
   MOZ_ASSERT(aItem);
   aItem->UpdateScrollData(&aOwner, this);
+  if (aTransform) {
+    // mTransform might have been set by the UpdateScrollData call above, so
+    // we should combine rather than clobber
+    mTransform = *aTransform * mTransform;
+  }
   for (const ActiveScrolledRoot* asr = aItem->GetActiveScrolledRoot();
        asr && asr != aStopAtAsr;
        asr = asr->mParent) {

gfx/layers/wr/WebRenderScrollData.h

@@ -47,7 +47,8 @@ public:
   void Initialize(WebRenderScrollData& aOwner,
                   nsDisplayItem* aItem,
                   int32_t aDescendantCount,
-                  const ActiveScrolledRoot* aStopAtAsr);
+                  const ActiveScrolledRoot* aStopAtAsr,
+                  const Maybe<gfx::Matrix4x4>& aTransform);
 
   int32_t GetDescendantCount() const;
   size_t GetScrollMetadataCount() const;