Enable experimental support for sub-pixel font scaling

Source/core/platform/graphics/FontFastPath.cpp

 /**
  * Copyright (C) 2003, 2006, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Holger Hans Peter Freyther
  * Copyright (C) 2009 Torch Mobile, Inc.
  *
  * 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
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 10 matching lines @@
  */
 
 #include "config.h"
 #include "core/platform/graphics/Font.h"
 
 #include "core/platform/graphics/FontCache.h"
 #include "core/platform/graphics/FontFallbackList.h"
 #include "core/platform/graphics/GlyphPageTreeNode.h"
 #include "core/platform/graphics/SimpleFontData.h"
 #include "core/platform/graphics/WidthIterator.h"
+#include "platform/LayoutUnit.h"
 #include "platform/fonts/GlyphBuffer.h"
 #include "platform/geometry/FloatRect.h"
 #include "platform/graphics/TextRun.h"
 #include "wtf/MainThread.h"
 #include "wtf/MathExtras.h"
 #include "wtf/unicode/CharacterNames.h"
 #include "wtf/unicode/Unicode.h"
 
 using namespace WTF;
 using namespace Unicode;
@@ skipping 508 matching lines @@
 
 FloatRect Font::selectionRectForSimpleText(const TextRun& run, const FloatPoint& point, int h, int from, int to) const
 {
     GlyphBuffer glyphBuffer;
     WidthIterator it(this, run);
     it.advance(from, &glyphBuffer);
     float beforeWidth = it.m_runWidthSoFar;
     it.advance(to, &glyphBuffer);
     float afterWidth = it.m_runWidthSoFar;
 
-    // Using roundf() rather than ceilf() for the right edge as a compromise to ensure correct caret positioning.
+    // Using roundf() rather than ceilf() for the right edge as a compromise to
+    // ensure correct caret positioning.
+    // Use LayoutUnit::epsilon() to ensure that values that cannot be stored as
+    // an integer are floored to n and not n-1 due to floating point imprecision.
     if (run.rtl()) {
         it.advance(run.length(), &glyphBuffer);
         float totalWidth = it.m_runWidthSoFar;
-        return FloatRect(floorf(point.x() + totalWidth - afterWidth), point.y(), roundf(point.x() + totalWidth - beforeWidth) - floorf(point.x() + totalWidth - afterWidth), h);
+        float pixelAlignedX = floorf(point.x() + totalWidth - afterWidth + LayoutUnit::epsilon());
+        return FloatRect(pixelAlignedX, point.y(),
+            roundf(point.x() + totalWidth - beforeWidth) - pixelAlignedX, h);
     }
 
-    return FloatRect(floorf(point.x() + beforeWidth), point.y(), roundf(point.x() + afterWidth) - floorf(point.x() + beforeWidth), h);
+    float pixelAlignedX = floorf(point.x() + beforeWidth + LayoutUnit::epsilon());
+    return FloatRect(pixelAlignedX, point.y(),
+        roundf(point.x() + afterWidth) - pixelAlignedX, h);
 }
 
 int Font::offsetForPositionForSimpleText(const TextRun& run, float x, bool includePartialGlyphs) const
 {
     float delta = x;
 
     WidthIterator it(this, run);
     GlyphBuffer localGlyphBuffer;
     unsigned offset;
     if (run.rtl()) {
@@ skipping 26 matching lines @@
                 if (delta <= 0)
                     break;
             }
         }
     }
 
     return offset;
 }
 
 }