Document how effective zoom relates to StyleImage sizing

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

 // Copyright 2015 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/NinePieceImagePainter.h"
 
 #include "core/frame/Deprecation.h"
 #include "core/layout/ImageQualityController.h"
 #include "core/layout/LayoutBoxModelObject.h"
 #include "core/paint/BoxPainter.h"
@@ skipping 31 matching lines @@
         || (style.borderTopWidth() && (style.borderTop().style() == BorderStyleNone || style.borderTop().style() == BorderStyleHidden))
         || (style.borderBottomWidth() && (style.borderBottom().style() == BorderStyleNone || style.borderBottom().style() == BorderStyleHidden)))
         Deprecation::countDeprecation(m_layoutObject.document(), UseCounter::BorderImageWithBorderStyleNone);
 
     // FIXME: border-image is broken with full page zooming when tiling has to happen, since the tiling function
     // doesn't have any understanding of the zoom that is in effect on the tile.
     LayoutRect rectWithOutsets = rect;
     rectWithOutsets.expand(style.imageOutsets(ninePieceImage));
     LayoutRect borderImageRect = rectWithOutsets;
 
+    // NinePieceImage returns the image slices without effective zoom applied and thus we compute
+    // the nine piece grid on top of the image in unzoomed coordinates.
     LayoutSize defaultObjectSize = borderImageRect.size();
     defaultObjectSize.scale(1 / style.effectiveZoom());
     IntSize imageSize = roundedIntSize(styleImage->imageSize(m_layoutObject, 1, defaultObjectSize));
 
     IntRectOutsets borderWidths(style.borderTopWidth(), style.borderRightWidth(),
         style.borderBottomWidth(), style.borderLeftWidth());
     NinePieceImageGrid grid(ninePieceImage, imageSize, pixelSnappedIntRect(borderImageRect), borderWidths);
 
     RefPtr<Image> image = styleImage->image(m_layoutObject, imageSize, 1);
 
@@ skipping 20 matching lines @@
                     drawInfo.tileRule.vertical, op);
             }
         }
     }
 
     graphicsContext.setImageInterpolationQuality(previousInterpolationQuality);
     return true;
 }
 
 } // namespace blink