Bug 1509272 - Always use new transform display items r=mattwoodrow

Differential Revision: https://phabricator.services.mozilla.com/D17425

--HG--
extra : moz-landing-system : lando

layout/painting/RetainedDisplayListBuilder.cpp

@@ -447,6 +447,11 @@ class MergeState {
       return true;
     }
 
+    if (type == DisplayItemType::TYPE_TRANSFORM) {
+      // Prerendering of transforms can change without frame invalidation.
+      return true;
+    }
+
     return false;
   }
 

layout/painting/nsDisplayList.cpp

@@ -4736,6 +4736,7 @@ void nsDisplayBackgroundColor::HitTest(nsDisplayListBuilder* aBuilder,
 void nsDisplayBackgroundColor::WriteDebugInfo(std::stringstream& aStream) {
   aStream << " (rgba " << mColor.r << "," << mColor.g << "," << mColor.b << ","
           << mColor.a << ")";
+  aStream << " backgroundRect" << mBackgroundRect;
 }
 
 already_AddRefed<Layer> nsDisplayClearBackground::BuildLayer(
@@ -8382,6 +8383,9 @@ void nsDisplayTransform::WriteDebugInfo(std::stringstream& aStream) {
   if (mFrame->Combines3DTransformWithAncestors()) {
     aStream << " combines-3d-with-ancestors";
   }
+
+  aStream << " allowAsync(" << (mAllowAsyncAnimation ? "true" : "false") << ")";
+  aStream << " childrenBuildingRect" << mChildrenBuildingRect;
 }
 
 nsDisplayPerspective::nsDisplayPerspective(nsDisplayListBuilder* aBuilder,