Fix for lack of cover with background-size: cover

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/LayoutView.h"
@@ skipping 91 matching lines @@
 
         // If the image has neither an intrinsic width nor an intrinsic height, its size is determined as for 'contain'.
         type = Contain;
     }
     case Contain:
     case Cover: {
         float horizontalScaleFactor = imageIntrinsicSize.width()
             ? positioningAreaSize.width().toFloat() / imageIntrinsicSize.width() : 1.0f;
         float verticalScaleFactor = imageIntrinsicSize.height()
             ? positioningAreaSize.height().toFloat() / imageIntrinsicSize.height() : 1.0f;
-        float scaleFactor = type == Contain ? std::min(horizontalScaleFactor, verticalScaleFactor) : std::max(horizontalScaleFactor, verticalScaleFactor);
-        return LayoutSize(std::max(1.0f, imageIntrinsicSize.width() * scaleFactor),
-            std::max(1.0f, imageIntrinsicSize.height() * scaleFactor));
+        // Force the dimension that determines the size to exactly match the
+        // positioningAreaSize in that dimension, so that rounding of floating point
+        // approximation to LayoutUnit do not shrink the image to smaller than the
+        // positioningAreaSize.

[Stephen Chennney, 2016/03/17 19:46:29]

Following the style guide makes the following code less obvious. It's really:

if (...) {
  if (...) {
  } else {
  }
} else {
  if (...) {
  } else {
  }
}

to cover the 4 cases: cover-x-bigger, cover-y-bigger, contain-x-smaller,
contain-y-smaller

+        if (type == Contain) {
+            if (horizontalScaleFactor < verticalScaleFactor)
+                return LayoutSize(positioningAreaSize.width(), LayoutUnit(std::max(1.0f, imageIntrinsicSize.height() * horizontalScaleFactor)));
+            return LayoutSize(LayoutUnit(std::max(1.0f, imageIntrinsicSize.width() * verticalScaleFactor)), positioningAreaSize.height());
+        }
+        if (horizontalScaleFactor > verticalScaleFactor)
+            return LayoutSize(positioningAreaSize.width(), LayoutUnit(std::max(1.0f, imageIntrinsicSize.height() * horizontalScaleFactor)));
+        return LayoutSize(LayoutUnit(std::max(1.0f, imageIntrinsicSize.width() * verticalScaleFactor)), positioningAreaSize.height());
     }
     }
 
     ASSERT_NOT_REACHED();
     return LayoutSize();
 }
 
 IntPoint accumulatedScrollOffsetForFixedBackground(const LayoutBoxModelObject& object, const LayoutBoxModelObject* container)
 {
     IntPoint result;
@@ skipping 223 matching lines @@
     // Clip the final output rect to the paint rect
     m_destRect.intersect(paintRect);
 
     // Snap as-yet unsnapped values.
     setPhase(LayoutPoint(roundedIntPoint(m_phase)));
     setDestRect(LayoutRect(pixelSnappedIntRect(m_destRect)));
 
 }
 
 } // namespace blink