Apply container offset and scroll between transform and container perspective.

third_party/WebKit/Source/core/layout/LayoutObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2004 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2011 Apple Inc.
  *               All rights reserved.
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved.
  *               (http://www.torchmobile.com/)
@@ skipping 2284 matching lines @@
   // transform directly.
   return (HasLayer() && ToLayoutBoxModelObject(this)->Layer()->Transform()) ||
          (container_object && container_object->Style()->HasPerspective());
 }
 
 void LayoutObject::GetTransformFromContainer(
     const LayoutObject* container_object,
     const LayoutSize& offset_in_container,
     TransformationMatrix& transform) const {
   transform.MakeIdentity();
-  transform.Translate(offset_in_container.Width().ToFloat(),
-                      offset_in_container.Height().ToFloat());
   PaintLayer* layer = HasLayer() ? ToLayoutBoxModelObject(this)->Layer() : 0;
   if (layer && layer->Transform())
     transform.Multiply(layer->CurrentTransform());
 
+  transform.TranslateRight(offset_in_container.Width().ToFloat(),

[chrishtr, 2017/04/10 19:37:35]

I can't come up with a case where the change here made a difference.
Don't know why - it should matter which order the transform and translation
happen in.

[trchen, 2017/04/10 21:15:18]

The code here is equivalent. See explanation above.

[chrishtr, 2017/04/10 21:38:28]

Ok.

+                           offset_in_container.Height().ToFloat());
+
   if (container_object && container_object->HasLayer() &&
       container_object->Style()->HasPerspective()) {
-    // Perpsective on the container affects us, so we have to factor it in here.
+    // Perspective on the container affects us, so we have to factor it in here.
     DCHECK(container_object->HasLayer());
     FloatPoint perspective_origin =
         ToLayoutBoxModelObject(container_object)->Layer()->PerspectiveOrigin();
 
     TransformationMatrix perspective_matrix;
     perspective_matrix.ApplyPerspective(
         container_object->Style()->Perspective());
+    perspective_matrix.ApplyTransformOrigin(perspective_origin.X(),
+                                            perspective_origin.Y(), 0);
 
-    transform.TranslateRight3d(-perspective_origin.X(), -perspective_origin.Y(),
-                               0);
+    // TODO(chrishtr): This applies perspective before the transform. Why?

[trchen, 2017/04/10 21:15:19]

This lgtm. It's TransformationMatrix.h lbtm. :/

[chrishtr, 2017/04/10 21:38:28]

Removed the comment.

     transform = perspective_matrix * transform;
-    transform.TranslateRight3d(perspective_origin.X(), perspective_origin.Y(),
-                               0);
   }
 }
 
 FloatQuad LayoutObject::LocalToAncestorQuad(
     const FloatQuad& local_quad,
     const LayoutBoxModelObject* ancestor,
     MapCoordinatesFlags mode) const {
   return LocalToAncestorQuadInternal(local_quad, ancestor,
                                      mode | kUseTransforms);
 }
@@ skipping 1311 matching lines @@
     const blink::LayoutObject* root = object1;
     while (root->Parent())
       root = root->Parent();
     root->ShowLayoutTreeAndMark(object1, "*", object2, "-", 0);
   } else {
     WTFLogAlways("%s", "Cannot showLayoutTree. Root is (nil)");
   }
 }
 
 #endif