Don't pass subpixel offsets through non-translation transforms

third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilderTest.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/PaintPropertyTreeBuilderTest.h"
 
 #include "core/html/HTMLIFrameElement.h"
 #include "core/layout/LayoutTreeAsText.h"
 #include "core/paint/ObjectPaintProperties.h"
 #include "core/paint/PaintPropertyTreePrinter.h"
@@ skipping 1749 matching lines @@
   // Visual rects via the non-paint properties system use enclosingIntRect
   // before applying transforms, because they are computed bottom-up and
   // therefore can't apply pixel snapping. Therefore apply a slop of 1px.
   CHECK_VISUAL_RECT(LayoutRect(LayoutUnit(0.7) + LayoutUnit(0.7),
                                LayoutUnit(0.7) + LayoutUnit(0.7),
                                LayoutUnit(40), LayoutUnit(40)),
                     c, frame_view->GetLayoutView(), 1);
 }
 
 TEST_P(PaintPropertyTreeBuilderTest,
+       NonTranslationTransformShouldResetSubpixelPaintOffset) {
+  SetBodyInnerHTML(
+      "<style>"
+      "  * { margin: 0; }"
+      "  div { position: relative; }"
+      "  #a {"
+      "    width: 70px;"
+      "    height: 70px;"
+      "    left: 0.8px;"
+      "    top: 0.8px;"
+      "  }"
+      "  #b {"
+      "    width: 40px;"
+      "    height: 40px;"
+      "    transform: scale(10);"
+      "    transform-origin: 0 0;"
+      "  }"
+      "  #c {"
+      "    width: 40px;"
+      "    height: 40px;"
+      "    left: 0.6px;"
+      "    top: 0.6px;"
+      "  }"
+      "</style>"
+      "<div id='a'>"
+      "  <div id='b'>"
+      "    <div id='c'></div>"
+      "  </div>"
+      "</div>");
+  FrameView* frame_view = GetDocument().View();
+
+  LayoutUnit a_offset = LayoutUnit(0.8);
+  LayoutUnit c_offset = LayoutUnit(0.6);
+
+  LayoutObject* b = GetDocument().getElementById("b")->GetLayoutObject();
+  const ObjectPaintProperties* b_properties = b->PaintProperties();
+  EXPECT_EQ(TransformationMatrix().Scale(10),
+            b_properties->Transform()->Matrix());
+  // The paint offset transform should not be snapped.
+  EXPECT_EQ(TransformationMatrix().Translate(a_offset.ToDouble(),
+                                             a_offset.ToDouble()),
+            b_properties->Transform()->Parent()->Matrix());
+  EXPECT_EQ(LayoutPoint(), b->PaintOffset());
+  CHECK_EXACT_VISUAL_RECT(
+      LayoutRect(a_offset, a_offset, LayoutUnit(400), LayoutUnit(400)), b,
+      frame_view->GetLayoutView());
+
+  // c's painting should start at c_offset.
+  LayoutObject* c = GetDocument().getElementById("c")->GetLayoutObject();
+  EXPECT_EQ(LayoutPoint(c_offset, c_offset), c->PaintOffset());
+  // Visual rects via the non-paint properties system use enclosingIntRect
+  // before applying transforms, because they are computed bottom-up and
+  // therefore can't apply pixel snapping. Therefore apply a slop of
+  // c_offset * 10 + 1px.
+  CHECK_VISUAL_RECT(
+      LayoutRect(c_offset * 10 + a_offset, c_offset * 10 + a_offset,
+                 LayoutUnit(400), LayoutUnit(400)),
+      c, frame_view->GetLayoutView(), c_offset * 10 + 1);
+}
+
+TEST_P(PaintPropertyTreeBuilderTest,
        PaintOffsetWithPixelSnappingThroughMultipleTransforms) {
   SetBodyInnerHTML(
       "<style>"
       "  * { margin: 0; }"
       "  div { position: relative; }"
       "  #a {"
       "    width: 70px;"
       "    height: 70px;"
       "    left: 0.7px;"
       "    top: 0.7px;"
@@ skipping 1690 matching lines @@
 
   LayoutObject& svg_root =
       *GetDocument().getElementById("svgroot")->GetLayoutObject();
   const ObjectPaintProperties* svg_root_properties = svg_root.PaintProperties();
   EXPECT_TRUE(svg_root_properties->Effect());
   EXPECT_EQ(EffectPaintPropertyNode::Root(),
             svg_root_properties->Effect()->Parent());
 }
 
 }  // namespace blink