Calculate positioningArea and not just size, for tiling background.

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 401 matching lines @@
 
 void BackgroundImageGeometry::calculate(
     const LayoutBoxModelObject& obj,
     const LayoutObject* backgroundObject,
     const LayoutBoxModelObject* paintContainer,
     const GlobalPaintFlags globalPaintFlags,
     const FillLayer& fillLayer,
     const LayoutRect& paintRect) {
   LayoutUnit left;
   LayoutUnit top;
-  LayoutSize positioningAreaSize;
+  LayoutRect positioningArea;
   bool isLayoutView = obj.isLayoutView();
   const LayoutBox* rootBox = nullptr;
   if (isLayoutView) {
     // It is only possible reach here when root element has a box.
     Element* documentElement = obj.document().documentElement();
     DCHECK(documentElement);
     DCHECK(documentElement->layoutObject());
     DCHECK(documentElement->layoutObject()->isBox());
     rootBox = toLayoutBox(documentElement->layoutObject());
   }
@@ skipping 41 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.
-      positioningAreaSize =
-          rootBox->size() - LayoutSize(left + right, top + bottom);
+      positioningArea.setSize(rootBox->size());
+      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 {
-      positioningAreaSize =
-          (cellUsingContainerBackground
-               ? getBackgroundObjectDimensions(toLayoutTableCell(obj),
-                                               toLayoutBox(positioningBox))
-               : paintRect.size()) -
-          LayoutSize(left + right, top + bottom);
+      if (cellUsingContainerBackground) {
+        positioningArea.setSize(getBackgroundObjectDimensions(
+            toLayoutTableCell(obj), toLayoutBox(positioningBox)));
+      } else {
+        positioningArea = paintRect;
+      }
+
+      positioningArea.contractEdges(top, right, bottom, left);
     }
   } else {
     setHasNonLocalGeometry();
 
     LayoutRect viewportRect = obj.viewRect();
     if (fixedBackgroundPaintsInLocalCoordinates(obj, globalPaintFlags)) {
       viewportRect.setLocation(LayoutPoint());
     } else {
       if (FrameView* frameView = obj.view()->frameView())
         viewportRect.setLocation(IntPoint(frameView->scrollOffsetInt()));
       // Compensate the translations created by ScrollRecorders.
       // TODO(trchen): Fix this for SP phase 2. crbug.com/529963.
       viewportRect.moveBy(
           accumulatedScrollOffsetForFixedBackground(obj, paintContainer));
     }
 
     if (paintContainer)
       viewportRect.moveBy(
           LayoutPoint(-paintContainer->localToAbsolute(FloatPoint())));
 
     setDestRect(viewportRect);
-    positioningAreaSize = destRect().size();
+    positioningArea = viewportRect;
   }
 
   LayoutSize fillTileSize(
-      calculateFillTileSize(positioningBox, fillLayer, positioningAreaSize));
+      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));
 
   EFillRepeat backgroundRepeatX = fillLayer.repeatX();
   EFillRepeat backgroundRepeatY = fillLayer.repeatY();
   LayoutUnit unsnappedAvailableWidth =
-      positioningAreaSize.width() - fillTileSize.width();
+      positioningArea.width() - fillTileSize.width();
   LayoutUnit unsnappedAvailableHeight =
-      positioningAreaSize.height() - fillTileSize.height();
-  positioningAreaSize =
-      LayoutSize(snapSizeToPixel(positioningAreaSize.width(), m_destRect.x()),
-                 snapSizeToPixel(positioningAreaSize.height(), m_destRect.y()));
+      positioningArea.height() - fillTileSize.height();
+  LayoutSize positioningAreaSize(positioningArea.pixelSnappedSize());

[Stephen Chennney, 2017/03/01 14:48:29]

This is a subtle change that might change snapping because we're no longer
trying to snap the same way that the dest rect will snap (or something like
that). Is your change in behavior deliberate? If not, leave it in the old format
in this case.

[Karl Øygard, 2017/03/03 15:00:07]

Good catch, this does not belong here.  Thanks!

   LayoutUnit availableWidth = positioningAreaSize.width() - tileSize().width();
   LayoutUnit availableHeight =
       positioningAreaSize.height() - tileSize().height();
 
   LayoutUnit computedXPosition =
       roundedMinimumValueForLength(fillLayer.xPosition(), availableWidth) -
       offsetInBackground.x();
   if (backgroundRepeatX == RoundFill &&
       positioningAreaSize.width() > LayoutUnit() &&
       fillTileSize.width() > LayoutUnit()) {
@@ skipping 90 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