Bug 1462672 - Part 5: Add a pref flag to allow flattening inactive nsDisplayTransform items r=mattwoodrow

MozReview-Commit-ID: BQGzaAMzReF

--HG--
extra : rebase_source : 42678409280a795fe891e10eceb86fa269d1d2de

gfx/thebes/gfxPrefs.h

@@ -683,6 +683,7 @@ private:
   DECL_GFX_PREF(Live, "layout.display-list.dump-content",      LayoutDumpDisplayListContent, bool, false);
   DECL_GFX_PREF(Live, "layout.display-list.dump-parent",       LayoutDumpDisplayListParent, bool, false);
   DECL_GFX_PREF(Live, "layout.display-list.show-rebuild-area", LayoutDisplayListShowArea, bool, false);
+  DECL_GFX_PREF(Live, "layout.display-list.flatten-transform", LayoutFlattenTransform, bool, true);
 
   DECL_GFX_PREF(Once, "layout.frame_rate",                     LayoutFrameRate, int32_t, -1);
   DECL_GFX_PREF(Once, "layout.less-event-region-items",        LessEventRegionItems, bool, true);

layout/painting/nsDisplayList.cpp

@@ -7637,6 +7637,7 @@ nsDisplayTransform::SetReferenceFrameToAncestor(nsDisplayListBuilder* aBuilder)
 void
 nsDisplayTransform::Init(nsDisplayListBuilder* aBuilder)
 {
+  mShouldFlatten = false;
   mHasBounds = false;
   mStoredList.SetClipChain(nullptr, true);
   mStoredList.SetBuildingRect(mChildrenBuildingRect);
@@ -7690,6 +7691,18 @@ nsDisplayTransform::nsDisplayTransform(nsDisplayListBuilder* aBuilder,
   UpdateBoundsFor3D(aBuilder);
 }
 
+bool
+nsDisplayTransform::ShouldFlattenAway(nsDisplayListBuilder* aBuilder)
+{
+  if (gfxVars::UseWebRender() || !gfxPrefs::LayoutFlattenTransform()) {
+    return false;
+  }
+
+  MOZ_ASSERT(!mShouldFlatten);
+  mShouldFlatten = GetTransform().Is2D();
+  return mShouldFlatten;
+}
+
 /* Returns the delta specified by the transform-origin property.
  * This is a positive delta, meaning that it indicates the direction to move
  * to get from (0, 0) of the frame to the transform origin.  This function is

layout/painting/nsDisplayList.h

@@ -6337,6 +6337,11 @@ public:
   }
 #endif
 
+  virtual void RestoreState() override
+  {
+    mShouldFlatten = false;
+  }
+
   virtual void UpdateBounds(nsDisplayListBuilder* aBuilder) override
   {
     mHasBounds = false;
@@ -6369,6 +6374,14 @@ public:
     return mStoredList.GetChildren();
   }
 
+  virtual RetainedDisplayList* GetSameCoordinateSystemChildren() const override
+  {
+    return mShouldFlatten ? mStoredList.GetChildren()
+                          : nullptr;
+  }
+
+  virtual bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) override;
+
   virtual void SetActiveScrolledRoot(const ActiveScrolledRoot* aActiveScrolledRoot) override
   {
     nsDisplayItem::SetActiveScrolledRoot(aActiveScrolledRoot);
@@ -6430,6 +6443,11 @@ public:
     }
   }
 
+  bool NeedsGeometryUpdates() const override
+  {
+    return mShouldFlatten;
+  }
+
   virtual const nsIFrame* ReferenceFrameForChildren() const override {
     // If we were created using a transform-getter, then we don't
     // belong to a transformed frame, and aren't a reference frame
@@ -6707,6 +6725,8 @@ private:
   bool mTransformPreserves3DInited;
   // True if async animation of the transform is allowed.
   bool mAllowAsyncAnimation;
+  // True if this nsDisplayTransform should get flattened
+  bool mShouldFlatten;
 };
 
 /* A display item that applies a perspective transformation to a single