Turn down BackgroundImageGeometry aggressive LayoutUnit conversion by a notch

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

 // Copyright 2014 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 "config.h"
 #include "core/paint/BackgroundImageGeometry.h"
 
 #include "core/frame/FrameView.h"
 #include "core/layout/LayoutBox.h"
 #include "core/layout/LayoutBoxModelObject.h"
@@ skipping 143 matching lines @@
 {
     LayoutPoint alignedPoint = attachmentPoint;
     m_phase.move(std::max(alignedPoint.x() - m_destRect.x(), LayoutUnit()), std::max(alignedPoint.y() - m_destRect.y(), LayoutUnit()));
 }
 
 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;
 }
 
 void BackgroundImageGeometry::pixelSnapGeometry()
 {
     setTileSize(applySubPixelHeuristicToImageSize(m_tileSize, m_destRect));
     setImageContainerSize(applySubPixelHeuristicToImageSize(m_imageContainerSize, m_destRect));
     setSpaceSize(LayoutSize(roundedIntSize(m_repeatSpacing)));
     setDestRect(LayoutRect(pixelSnappedIntRect(m_destRect)));
     setPhase(LayoutPoint(flooredIntPoint(m_phase)));
@@ skipping 76 matching lines @@
             viewportRect.moveBy(accumulatedScrollOffset(obj, paintContainer));
         }
 
         if (paintContainer)
             viewportRect.moveBy(LayoutPoint(-paintContainer->localToAbsolute(FloatPoint())));
 
         setDestRect(viewportRect);
         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);
 
     EFillRepeat backgroundRepeatX = fillLayer.repeatX();
     EFillRepeat backgroundRepeatY = fillLayer.repeatY();
     LayoutUnit availableWidth = positioningAreaSize.width() - tileSize().width();
     LayoutUnit availableHeight = positioningAreaSize.height() - tileSize().height();
 
     LayoutUnit computedXPosition = roundedMinimumValueForLength(fillLayer.xPosition(), availableWidth);
     if (backgroundRepeatX == RoundFill && positioningAreaSize.width() > LayoutUnit() && fillTileSize.width() > LayoutUnit()) {
@@ skipping 74 matching lines @@
     }
 
     if (fixedAttachment)
         useFixedAttachment(paintRect.location());
 
     clip(paintRect);
     pixelSnapGeometry();
 }
 
 } // namespace blink