Fixed rounding issue on scrollbar rasterization.

cc/layers/painted_scrollbar_layer.cc

Index: cc/layers/painted_scrollbar_layer.cc
diff --git a/cc/layers/painted_scrollbar_layer.cc b/cc/layers/painted_scrollbar_layer.cc
index 71a37d48883f8f07d592ff561e288e344917ed65..80e5f1b6523f59e7c20eca8113f98467b9e71ca9 100644
--- a/cc/layers/painted_scrollbar_layer.cc
+++ b/cc/layers/painted_scrollbar_layer.cc
@@ -170,7 +170,7 @@ gfx::Rect PaintedScrollbarLayer::ScrollbarLayerRectToContentRect(
   // Don't intersect with the bounds as in LayerRectToContentRect() because
   // layer_rect here might be in coordinates of the containing layer.
   gfx::Rect expanded_rect = gfx::ScaleToEnclosingRect(
-      layer_rect, contents_scale_y(), contents_scale_y());
+      layer_rect, contents_scale_x(), contents_scale_y());

[bokan, 2014/02/11 00:01:53]

Not that it matters since scale_x == scale_y, but I assume this was a typo?

   // We should never return a rect bigger than the content_bounds().
   gfx::Size clamped_size = expanded_rect.size();
   clamped_size.SetToMin(content_bounds());
@@ -187,7 +187,7 @@ gfx::Rect PaintedScrollbarLayer::OriginThumbRect() const {
     thumb_size =
         gfx::Size(scrollbar_->ThumbThickness(), scrollbar_->ThumbLength());
   }
-  return ScrollbarLayerRectToContentRect(gfx::Rect(thumb_size));
+  return gfx::Rect(thumb_size);
 }
 
 void PaintedScrollbarLayer::UpdateThumbAndTrackGeometry() {
@@ -205,8 +205,9 @@ bool PaintedScrollbarLayer::Update(ResourceUpdateQueue* queue,
                                    const OcclusionTracker* occlusion) {
   UpdateThumbAndTrackGeometry();
 
+  gfx::Rect track_layer_rect = gfx::Rect(location_, bounds());
   gfx::Rect scaled_track_rect = ScrollbarLayerRectToContentRect(
-      gfx::Rect(location_, bounds()));
+      track_layer_rect);
 
   if (track_rect_.IsEmpty() || scaled_track_rect.IsEmpty())
     return false;
@@ -221,12 +222,16 @@ bool PaintedScrollbarLayer::Update(ResourceUpdateQueue* queue,
     return false;
 
   track_resource_ = ScopedUIResource::Create(
-      layer_tree_host(), RasterizeScrollbarPart(scaled_track_rect, TRACK));
+      layer_tree_host(),
+      RasterizeScrollbarPart(track_layer_rect, scaled_track_rect, TRACK));
 
-  gfx::Rect thumb_rect = OriginThumbRect();
-  if (has_thumb_ && !thumb_rect.IsEmpty()) {
+  gfx::Rect thumb_layer_rect = OriginThumbRect();
+  gfx::Rect scaled_thumb_rect =
+      ScrollbarLayerRectToContentRect(thumb_layer_rect);
+  if (has_thumb_ && !scaled_thumb_rect.IsEmpty()) {
     thumb_resource_ = ScopedUIResource::Create(
-        layer_tree_host(), RasterizeScrollbarPart(thumb_rect, THUMB));
+        layer_tree_host(),
+        RasterizeScrollbarPart(thumb_layer_rect, scaled_thumb_rect, THUMB));
   }
 
   // UI resources changed so push properties is needed.
@@ -235,21 +240,26 @@ bool PaintedScrollbarLayer::Update(ResourceUpdateQueue* queue,
 }
 
 UIResourceBitmap PaintedScrollbarLayer::RasterizeScrollbarPart(
+    const gfx::Rect& layer_rect,
     const gfx::Rect& rect,
     ScrollbarPart part) {
   DCHECK(!rect.size().IsEmpty());
+  DCHECK(!layer_rect.size().IsEmpty());
 
   SkBitmap skbitmap;
   skbitmap.setConfig(SkBitmap::kARGB_8888_Config, rect.width(), rect.height());
   skbitmap.allocPixels();
 
   SkCanvas skcanvas(skbitmap);
-  skcanvas.translate(SkFloatToScalar(-rect.x()), SkFloatToScalar(-rect.y()));
+
+  // Skia will round to the nearest pixel so we need to translate by the
+  // rounded origin coordinates
+  float roundedX = round(layer_rect.x() * contents_scale_x());
+  float roundedY = round(layer_rect.y() * contents_scale_y());
+  skcanvas.translate(SkFloatToScalar(-roundedX), SkFloatToScalar(-roundedY));
   skcanvas.scale(SkFloatToScalar(contents_scale_x()),
                  SkFloatToScalar(contents_scale_y()));
 
-  gfx::Rect layer_rect = gfx::ScaleToEnclosingRect(
-      rect, 1.f / contents_scale_x(), 1.f / contents_scale_y());
   SkRect layer_skrect = RectToSkRect(layer_rect);
   SkPaint paint;
   paint.setAntiAlias(false);