Turn down BackgroundImageGeometry aggressive LayoutUnit conversion by a notch

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

Index: third_party/WebKit/Source/core/paint/BackgroundImageGeometry.cpp
diff --git a/third_party/WebKit/Source/core/paint/BackgroundImageGeometry.cpp b/third_party/WebKit/Source/core/paint/BackgroundImageGeometry.cpp
index ef37cf3ece731dcf97b5bd801b738181c5decaba..0bd22707697bebe0e3a489634ec7a68eef699a60 100644
--- a/third_party/WebKit/Source/core/paint/BackgroundImageGeometry.cpp
+++ b/third_party/WebKit/Source/core/paint/BackgroundImageGeometry.cpp
@@ -161,14 +161,15 @@ void BackgroundImageGeometry::clip(const LayoutRect& clipRect)
     m_destRect.intersect(clipRect);
 }
 
-// When we match the destination rect in a dimension, we snap the same way. Otherwise
-// we floor to avoid growing our tile size. Often these tiles are from a sprite map,
-// and bleeding adjactent sprites is visually worse than clipping the intenteded one.
+// When we match the sub-pixel fraction of the destination rect in a dimension, we
+// snap the same way. Otherwise we floor to avoid growing our tile size. Often these
+// tiles are from a sprite map, and bleeding adjactent sprites is visually worse
+// than clipping the intenteded one.
 static LayoutSize applySubPixelHeuristicToImageSize(const LayoutSize& size, const LayoutRect& destination)

[davve, 2015/12/08 20:19:21]

nit: If you like you can put this function in the unnamed namespace, as is done
for the other helper functions in this file.

 {
     LayoutSize snappedSize = LayoutSize(
-        size.width() == destination.width() ? destination.pixelSnappedWidth() : size.width().floor(),
-        size.height() == destination.height() ? destination.pixelSnappedHeight() : size.height().floor());
+        size.width().fraction() == destination.width().fraction() ? snapSizeToPixel(size.width(), destination.x()) : size.width().floor(),

[davve, 2015/12/08 20:19:21]

The other change below is easier to understand than this one. What was the
reason for checking only the fraction part?

+        size.height().fraction() == destination.height().fraction() ? snapSizeToPixel(size.height(), destination.y()) : size.height().floor());
     return snappedSize;
 }
 
@@ -265,7 +266,10 @@ void BackgroundImageGeometry::calculate(const LayoutBoxModelObject& obj, const L
         positioningAreaSize = destRect().size();
     }
 
-    LayoutSize fillTileSize = calculateFillTileSize(positioningBox, fillLayer, positioningAreaSize);
+    LayoutSize fillTileSize(calculateFillTileSize(positioningBox, fillLayer, positioningAreaSize));
+    // It's necessary to apply the heuristic here prior to any further calculations to avoid
+    // incorrectly using sub-pixel values that won't be present in the painted tile.
+    fillTileSize = applySubPixelHeuristicToImageSize(fillTileSize, m_destRect);
     setTileSize(fillTileSize);
     setImageContainerSize(fillTileSize);