Don't pixel-snap overflow clips in clip paint property nodes.

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

Index: third_party/WebKit/Source/core/paint/PaintLayerClipper.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerClipper.cpp b/third_party/WebKit/Source/core/paint/PaintLayerClipper.cpp
index 96fe94dd115a0cc4288aa04e12de721f860e13bf..4ccfc42bf08430bf43c814a9617e140299688a95 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerClipper.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerClipper.cpp
@@ -249,8 +249,16 @@ LayoutRect PaintLayerClipper::localClipRect(
 
 #ifdef CHECK_CLIP_RECTS
 #define CHECK_RECTS_EQ(expected, actual)                                \
-  CHECK((expected.isEmpty() && actual.isEmpty()) || expected == actual) \
-      << "expected=" << expected.toString() << " actual=" << actual.toString()
+  do {                                                                  \

[chrishtr, 2017/02/16 21:44:28]

I adjusted this to not assert, in order to get better debugging output.
Regardless, the checks are not on in debug or any other compile mode
by default.

+    bool matches =                                                      \
+        (expected.isEmpty() && actual.isEmpty()) || expected == actual; \
+    if (!matches) {                                                     \
+      LOG(ERROR) << "Rects don't match for m_layer="                    \
+                 << m_layer.layoutObject()->debugName()                 \
+                 << " expected=" << expected.toString()                 \
+                 << " actual=" << actual.toString();                    \
+    }                                                                   \
+  } while (false);
 #endif
 
 void PaintLayerClipper::mapLocalToRootWithGeometryMapper(
@@ -272,6 +280,7 @@ void PaintLayerClipper::mapLocalToRootWithGeometryMapper(
   rectToMap = LayoutRect(m_geometryMapper->sourceToDestinationRect(
       localRect, layerTransform, rootTransform));
   rectToMap.moveBy(-context.rootLayer->layoutObject()->paintOffset());
+  rectToMap.move(context.subPixelAccumulation);

[chrishtr, 2017/02/16 21:44:28]

This is exercised by the layout test
fast/sub-pixel/fractional-border-location-and-size.html in addition to the
new unittest.

 }
 
 void PaintLayerClipper::calculateRectsWithGeometryMapper(
@@ -288,16 +297,15 @@ void PaintLayerClipper::calculateRectsWithGeometryMapper(
     foregroundRect = ClipRect(LayoutRect(LayoutRect::infiniteIntRect()));
   } else {
     backgroundRect = clipRectWithGeometryMapper(context, false);
-    applyOverflowClipToBackgroundRectWithGeometryMapper(context,
-                                                        backgroundRect);
 
-    backgroundRect.move(
-        context.subPixelAccumulation);  // TODO(chrishtr): is this needed?

[chrishtr, 2017/02/16 21:44:28]

Confirmed that it is needed.

+    backgroundRect.move(context.subPixelAccumulation);
     backgroundRect.intersect(paintDirtyRect);
 
-    foregroundRect.move(
-        context.subPixelAccumulation);  // TODO(chrishtr): is this needed?
+    applyOverflowClipToBackgroundRectWithGeometryMapper(context,
+                                                        backgroundRect);
+
     foregroundRect = clipRectWithGeometryMapper(context, true);
+    foregroundRect.move(context.subPixelAccumulation);

[chrishtr, 2017/02/16 21:44:28]

I moved this above new line 309 for consistency with the old code.

     foregroundRect.intersect(paintDirtyRect);
   }
   LayoutPoint offset;
@@ -310,9 +318,9 @@ void PaintLayerClipper::calculateRectsWithGeometryMapper(
 #ifdef CHECK_CLIP_RECTS
   ClipRect testBackgroundRect, testForegroundRect;
   LayoutRect testLayerBounds;
-  PaintLayerClipper(m_layer, false)
+  PaintLayerClipper(m_layer, nullptr)
       .calculateRects(context, paintDirtyRect, testLayerBounds,
-                      testBackgroundRect, testForegroundRect);
+                      testBackgroundRect, testForegroundRect, offsetFromRoot);
   CHECK_RECTS_EQ(testBackgroundRect, backgroundRect);
   CHECK_RECTS_EQ(testForegroundRect, foregroundRect);
   CHECK_RECTS_EQ(testLayerBounds, layerBounds);
@@ -510,7 +518,7 @@ ClipRect PaintLayerClipper::backgroundClipRect(
     ClipRect backgroundClipRect = clipRectWithGeometryMapper(context, false);
 #ifdef CHECK_CLIP_RECTS
     ClipRect testBackgroundClipRect =
-        PaintLayerClipper(m_layer, false).backgroundClipRect(context);
+        PaintLayerClipper(m_layer, nullptr).backgroundClipRect(context);
     CHECK_RECTS_EQ(testBackgroundClipRect, backgroundClipRect);
 #endif
     return backgroundClipRect;