Fix 'text-underline-position: under' to use em height ascent/descent

third_party/WebKit/Source/platform/fonts/SimpleFontData.h

 /*
  * This file is part of the internal font implementation.
  *
  * Copyright (C) 2006, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2007-2008 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 100 matching lines @@
   FontMetrics& GetFontMetrics() { return font_metrics_; }
   const FontMetrics& GetFontMetrics() const { return font_metrics_; }
   float SizePerUnit() const {
     return PlatformData().size() /
            (GetFontMetrics().UnitsPerEm() ? GetFontMetrics().UnitsPerEm() : 1);
   }
   float InternalLeading() const {
     return GetFontMetrics().FloatHeight() - PlatformData().size();
   }
 
+  // "em height" metrics.
+  // https://drafts.css-houdini.org/font-metrics-api-1/#fontmetrics
+  LayoutUnit EmHeightAscent(FontBaseline = kAlphabeticBaseline) const;
+  LayoutUnit EmHeightDescent(FontBaseline = kAlphabeticBaseline) const;
+
   float MaxCharWidth() const { return max_char_width_; }
   void SetMaxCharWidth(float max_char_width) {
     max_char_width_ = max_char_width;
   }
 
   float AvgCharWidth() const { return avg_char_width_; }
   void SetAvgCharWidth(float avg_char_width) {
     avg_char_width_ = avg_char_width;
   }
 
@@ skipping 49 matching lines @@
   // Only used for testing.
   SimpleFontData(const FontPlatformData&, PassRefPtr<OpenTypeVerticalData>);
 
  private:
   void PlatformInit(bool subpixel_ascent_descent);
   void PlatformGlyphInit();
 
   PassRefPtr<SimpleFontData> CreateScaledFontData(const FontDescription&,
                                                   float scale_factor) const;
 
+  void ComputeEmHeightMetrics() const;
+  bool NormalizeEmHeightMetrics(float, float) const;
+
   FontMetrics font_metrics_;
   float max_char_width_;
   float avg_char_width_;
 
   FontPlatformData platform_data_;
   SkPaint paint_;
 
   RefPtr<OpenTypeVerticalData> vertical_data_;
 
   Glyph space_glyph_;
@@ skipping 24 matching lines @@
 
   unsigned is_text_orientation_fallback_ : 1;
   unsigned has_vertical_glyphs_ : 1;
 
   // These are set to non-zero when ascent or descent is rounded or shifted
   // to be smaller than the actual ascent or descent. When calculating visual
   // overflows, we should add the inflations.
   unsigned visual_overflow_inflation_for_ascent_ : 2;
   unsigned visual_overflow_inflation_for_descent_ : 2;
 
+  mutable LayoutUnit em_height_ascent_;
+  mutable LayoutUnit em_height_descent_;
+
 // See discussion on crbug.com/631032 and Skiaissue
 // https://bugs.chromium.org/p/skia/issues/detail?id=5328 :
 // On Mac we're still using path based glyph metrics, and they seem to be
 // too slow to be able to remove the caching layer we have here.
 #if OS(MACOSX)
   mutable std::unique_ptr<GlyphMetricsMap<FloatRect>> glyph_to_bounds_map_;
   mutable GlyphMetricsMap<float> glyph_to_width_map_;
 #endif
 };
 
@@ skipping 29 matching lines @@
 
   glyph_to_width_map_.SetMetricsForGlyph(glyph, width);
   return width;
 #endif
 }
 
 DEFINE_FONT_DATA_TYPE_CASTS(SimpleFontData, false);
 
 }  // namespace blink
 #endif  // SimpleFontData_h