Use plus/minus 0.5 instead of 1 when calculating zoomed length.

Source/core/rendering/style/RenderStyle.h

 /*
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 1770 matching lines @@
 
     void appendContent(PassOwnPtr<ContentData>);
 
     bool diffNeedsFullLayout(const RenderStyle& other) const;
     bool diffNeedsRepaintLayerOnly(const RenderStyle& other) const;
     bool diffNeedsRepaintObjectOnly(const RenderStyle& other) const;
     bool diffNeedsRecompositeLayerOnly(const RenderStyle& other) const;
     unsigned computeChangedContextSensitiveProperties(const RenderStyle& other, StyleDifference) const;
 };
 
 inline int adjustForAbsoluteZoom(int value, float zoomFactor)

[eae, 2014/04/28 14:44:48]

We really shouldn't be using ints for layout values at all and the long term fix
here would likely be to use the float or LayoutUnit versions instead,

As a short term intermix fix, however, I suppose this does make more sense.

Could you please add a FIXME to reduce/remove our dependency on zoom adjusted
int values?

 {
     if (zoomFactor == 1)
         return value;
     // Needed because computeLengthInt truncates (rather than rounds) when scaling up.
+    float fvalue = value;
     if (zoomFactor > 1) {
         if (value < 0)
-            value--;
+            fvalue -= 0.5f;
         else
-            value++;
+            fvalue += 0.5f;
     }
 
-    return roundForImpreciseConversion<int>(value / zoomFactor);
+    return roundForImpreciseConversion<int>(fvalue / zoomFactor);
 }
 
 inline int adjustForAbsoluteZoom(int value, const RenderStyle* style)
 {
     return adjustForAbsoluteZoom(value, style->effectiveZoom());
 }
 
 inline float adjustFloatForAbsoluteZoom(float value, const RenderStyle& style)
 {
     return value / style.effectiveZoom();
@@ skipping 61 matching lines @@
 inline bool RenderStyle::hasPseudoElementStyle() const
 {
     return noninherited_flags._pseudoBits & PSEUDO_ELEMENT_MASK;
 }
 
 float calcBorderRadiiConstraintScaleFor(const FloatRect&, const FloatRoundedRect::Radii&);
 
 } // namespace WebCore
 
 #endif // RenderStyle_h