Don't round x-axis for text runs when subpixel font scaling is enabled

Source/core/rendering/InlineTextBox.cpp

 /*
  * (C) 1999 Lars Knoll (knoll@kde.org)
  * (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * 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 460 matching lines @@
     ASSERT(paintInfo.phase != PaintPhaseSelfOutline && paintInfo.phase != PaintPhaseChildOutlines);
 
     LayoutUnit logicalLeftSide = logicalLeftVisualOverflow();
     LayoutUnit logicalRightSide = logicalRightVisualOverflow();
     LayoutUnit logicalStart = logicalLeftSide + (isHorizontal() ? paintOffset.x() : paintOffset.y());
     LayoutUnit logicalExtent = logicalRightSide - logicalLeftSide;
 
     LayoutUnit paintEnd = isHorizontal() ? paintInfo.rect.maxX() : paintInfo.rect.maxY();
     LayoutUnit paintStart = isHorizontal() ? paintInfo.rect.x() : paintInfo.rect.y();
 
-    LayoutPoint adjustedPaintOffset = roundedIntPoint(paintOffset);
+    // When subpixel font scaling is enabled text runs are positioned at
+    // subpixel boundaries on the x-axis and thus there is no reason to
+    // snap the x value. We still round the y-axis to ensure consistent
+    // line heights.
+    LayoutPoint adjustedPaintOffset = RuntimeEnabledFeatures::subpixelFontScalingEnabled()
+        ? LayoutPoint(paintOffset.x(), paintOffset.y().round())
+        : roundedIntPoint(paintOffset);
 
     if (logicalStart >= paintEnd || logicalStart + logicalExtent <= paintStart)
         return;
 
     bool isPrinting = textRenderer().document().printing();
 
     // Determine whether or not we're selected.
     bool haveSelection = !isPrinting && paintInfo.phase != PaintPhaseTextClip && selectionState() != RenderObject::SelectionNone;
     if (!haveSelection && paintInfo.phase == PaintPhaseSelection)
         // When only painting the selection, don't bother to paint if there is none.
@@ skipping 18 matching lines @@
     }
 
     GraphicsContext* context = paintInfo.context;
 
     RenderObject& rendererToUse = renderer();
     RenderStyle* styleToUse = rendererToUse.style(isFirstLineStyle());
 
     adjustedPaintOffset.move(0, styleToUse->isHorizontalWritingMode() ? 0 : -logicalHeight());
 
     FloatPoint boxOrigin = locationIncludingFlipping();
-    // FIXME: Shouldn't these offsets be rounded?
     boxOrigin.move(adjustedPaintOffset.x().toFloat(), adjustedPaintOffset.y().toFloat());
     FloatRect boxRect(boxOrigin, LayoutSize(logicalWidth(), logicalHeight()));
 
     RenderCombineText* combinedText = styleToUse->hasTextCombine() && textRenderer().isCombineText() && toRenderCombineText(textRenderer()).isCombined() ? &toRenderCombineText(textRenderer()) : 0;
 
     bool shouldRotate = !isHorizontal() && !combinedText;
     if (shouldRotate)
         context->concatCTM(rotation(boxRect, Clockwise));
 
     // Determine whether or not we have composition underlines to draw.
@@ skipping 1043 matching lines @@
     printedCharacters = fprintf(stderr, "\t%s %p", obj.renderName(), &obj);
     const int rendererCharacterOffset = 24;
     for (; printedCharacters < rendererCharacterOffset; printedCharacters++)
         fputc(' ', stderr);
     fprintf(stderr, "(%d,%d) \"%s\"\n", start(), start() + len(), value.utf8().data());
 }
 
 #endif
 
 } // namespace WebCore