Calculate Underline thickness from Font

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 1066 matching lines @@
     bool isPrinting = textRenderer()->document().printing();
 
     bool linesAreOpaque = !isPrinting && (!(deco & TextDecorationUnderline) || underline.alpha() == 255) && (!(deco & TextDecorationOverline) || overline.alpha() == 255) && (!(deco & TextDecorationLineThrough) || linethrough.alpha() == 255);
 
     RenderStyle* styleToUse = renderer()->style(isFirstLineStyle());
     int baseline = styleToUse->fontMetrics().ascent();
 
     size_t shadowCount = shadowList ? shadowList->shadows().size() : 0;
     // Set the thick of the line to be 10% (or something else ?)of the computed font size and not less than 1px.
     // Using computedFontSize should take care of zoom as well.
-    const float textDecorationThickness = std::max(1.f, styleToUse->computedFontSize() / 10.f);
+
+    // Update Underline thickness, in case we have Faulty Font Metrics calculating underline thickness by old method.
+    float textDecorationThickness = styleToUse->fontMetrics().underlineThickness();
+    int fontHeightInt  = (int)(styleToUse->fontMetrics().floatHeight() + 0.5);
+    if ((textDecorationThickness == 0.f) || (textDecorationThickness >= (fontHeightInt >> 1)))
+        textDecorationThickness = std::max(1.f, styleToUse->computedFontSize() / 10.f);
+
     context->setStrokeThickness(textDecorationThickness);
 
     float extraOffset = 0;
     if (!linesAreOpaque && shadowCount > 1) {
         FloatRect clipRect(localOrigin, FloatSize(width, baseline + 2));
         for (size_t i = shadowCount; i--; ) {
             const ShadowData& s = shadowList->shadows()[i];
             FloatRect shadowRect(localOrigin, FloatSize(width, baseline + 2));
             shadowRect.inflate(s.blur());
             float shadowX = isHorizontal() ? s.x() : s.y();
@@ skipping 458 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