Account for borders and padding when calculating background tile size.

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 "core/paint/BackgroundImageGeometry.h"
 
 #include "core/frame/FrameView.h"
 #include "core/layout/LayoutBox.h"
 #include "core/layout/LayoutBoxModelObject.h"
 #include "core/layout/LayoutTableCell.h"
@@ skipping 463 matching lines @@
         right += positioningBox.paddingRight();
         top += positioningBox.paddingTop();
         bottom += positioningBox.paddingBottom();
       }
     }
 
     if (isLayoutView) {
       // The background of the box generated by the root element covers the
       // entire canvas and will be painted by the view object, but the we should
       // still use the root element box for positioning.
       positioningArea.setSize(rootBox->size());

[Stephen Chennney, 2017/02/13 20:24:00]

Do we have a test case that hits this code path? This is the only place I see
where the positioningArea may be very different from the paint rect that we
otherwise would use.

       positioningArea.contractEdges(top, right, bottom, left);
 
       // The input paint rect is specified in root element local coordinate
       // (i.e. a transform is applied on the context for painting), and is
       // expanded to cover the whole canvas.  Since left/top is relative to the
       // paint rect, we need to offset them back.
       left -= paintRect.x();
       top -= paintRect.y();
     } else {
       if (cellUsingContainerBackground) {
@@ skipping 26 matching lines @@
 
     setDestRect(viewportRect);
     positioningArea = viewportRect;
   }
 
   LayoutSize fillTileSize(
       calculateFillTileSize(positioningBox, fillLayer, positioningArea.size()));
   // 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.
-  setTileSize(applySubPixelHeuristicToImageSize(fillTileSize, m_destRect));
+  setTileSize(applySubPixelHeuristicToImageSize(fillTileSize, positioningArea));
 
   EFillRepeat backgroundRepeatX = fillLayer.repeatX();
   EFillRepeat backgroundRepeatY = fillLayer.repeatY();
   LayoutUnit unsnappedAvailableWidth =
       positioningArea.width() - fillTileSize.width();
   LayoutUnit unsnappedAvailableHeight =
       positioningArea.height() - fillTileSize.height();
   LayoutSize positioningAreaSize(positioningArea.pixelSnappedSize());
   LayoutUnit availableWidth = positioningAreaSize.width() - tileSize().width();
   LayoutUnit availableHeight =
@@ skipping 10 matching lines @@
     LayoutUnit roundedWidth = positioningAreaSize.width() / nrTiles;
 
     // Maintain aspect ratio if background-size: auto is set
     if (fillLayer.size().size.height().isAuto() &&
         backgroundRepeatY != RoundFill) {
       fillTileSize.setHeight(fillTileSize.height() * roundedWidth /
                              fillTileSize.width());
     }
     fillTileSize.setWidth(roundedWidth);
 
-    setTileSize(applySubPixelHeuristicToImageSize(fillTileSize, m_destRect));
+    setTileSize(
+        applySubPixelHeuristicToImageSize(fillTileSize, positioningArea));
     setPhaseX(tileSize().width()
                   ? LayoutUnit(roundf(
                         tileSize().width() -
                         fmodf((computedXPosition + left), tileSize().width())))
                   : LayoutUnit());
     setSpaceSize(LayoutSize());
   }
 
   LayoutUnit computedYPosition =
       roundedMinimumValueForLength(fillLayer.yPosition(), availableHeight) -
       offsetInBackground.y();
   if (backgroundRepeatY == RoundFill &&
       positioningAreaSize.height() > LayoutUnit() &&
       fillTileSize.height() > LayoutUnit()) {
     int nrTiles = std::max(
         1, roundToInt(positioningAreaSize.height() / fillTileSize.height()));
     LayoutUnit roundedHeight = positioningAreaSize.height() / nrTiles;
     // Maintain aspect ratio if background-size: auto is set
     if (fillLayer.size().size.width().isAuto() &&
         backgroundRepeatX != RoundFill) {
       fillTileSize.setWidth(fillTileSize.width() * roundedHeight /
                             fillTileSize.height());
     }
     fillTileSize.setHeight(roundedHeight);
 
-    setTileSize(applySubPixelHeuristicToImageSize(fillTileSize, m_destRect));
+    setTileSize(
+        applySubPixelHeuristicToImageSize(fillTileSize, positioningArea));
     setPhaseY(tileSize().height()
                   ? LayoutUnit(roundf(
                         tileSize().height() -
                         fmodf((computedYPosition + top), tileSize().height())))
                   : LayoutUnit());
     setSpaceSize(LayoutSize());
   }
 
   if (backgroundRepeatX == RepeatFill) {
     setRepeatX(fillLayer, fillTileSize.width(), availableWidth,
@@ skipping 41 matching lines @@
     useFixedAttachment(paintRect.location());
 
   // Clip the final output rect to the paint rect
   m_destRect.intersect(paintRect);
 
   // Snap as-yet unsnapped values.
   setDestRect(LayoutRect(pixelSnappedIntRect(m_destRect)));
 }
 
 }  // namespace blink