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 393 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 =
+      LayoutSize(snapSizeToPixel(positioningArea.width(), m_destRect.x()),
+                 snapSizeToPixel(positioningArea.height(), m_destRect.y()));
   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