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

third_party/WebKit/Source/core/layout/line/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 107 matching lines @@
     return LayoutUnit(
         LineLayoutBR(GetLineLayoutItem()).LineHeight(IsFirstLineStyle()));
   if (Parent()->GetLineLayoutItem() == GetLineLayoutItem().Parent())
     return Parent()->LineHeight();
   return LineLayoutBoxModel(GetLineLayoutItem().Parent())
       .LineHeight(IsFirstLineStyle(),
                   IsHorizontal() ? kHorizontalLine : kVerticalLine,
                   kPositionOnContainingLine);
 }
 
+LayoutUnit InlineTextBox::OffsetTo(LineVerticalPositionType position_type,
+                                   FontBaseline baseline_type) const {
+  if (IsText() &&
+      (position_type == LineVerticalPositionType::TopOfEmHeight ||
+       position_type == LineVerticalPositionType::BottomOfEmHeight)) {
+    const Font& font = GetLineLayoutItem().Style(IsFirstLineStyle())->GetFont();
+    if (const SimpleFontData* font_data = font.PrimaryFont()) {
+      const FontMetrics& metrics = font_data->GetFontMetrics();
+      if (position_type == LineVerticalPositionType::TopOfEmHeight) {
+        // Use Ascent, not FixedAscent, to match to how InlineTextBoxPainter
+        // computes the baseline position.
+        return metrics.Ascent(baseline_type) -
+               font_data->EmHeightAscent(baseline_type);
+      }
+      if (position_type == LineVerticalPositionType::BottomOfEmHeight) {
+        return metrics.Ascent(baseline_type) +
+               font_data->EmHeightDescent(baseline_type);
+      }
+    }
+  }
+  switch (position_type) {
+    case LineVerticalPositionType::TextTop:
+    case LineVerticalPositionType::TopOfEmHeight:
+      return LayoutUnit();
+    case LineVerticalPositionType::TextBottom:
+    case LineVerticalPositionType::BottomOfEmHeight:
+      return LogicalHeight();
+  }
+  NOTREACHED();
+  return LayoutUnit();
+}
+
+LayoutUnit InlineTextBox::VerticalPosition(
+    LineVerticalPositionType position_type,
+    FontBaseline baseline_type) const {
+  return LogicalTop() + OffsetTo(position_type, baseline_type);
+}
+
 bool InlineTextBox::IsSelected(int start_pos, int end_pos) const {
   int s_pos = std::max(start_pos - start_, 0);
   // The position after a hard line break is considered to be past its end.
   // See the corresponding code in InlineTextBox::getSelectionState.
   int e_pos = std::min(end_pos - start_, int(len_) + (IsLineBreak() ? 0 : 1));
   return (s_pos < e_pos);
 }
 
 SelectionState InlineTextBox::GetSelectionState() const {
   SelectionState state = GetLineLayoutItem().GetSelectionState();
@@ skipping 584 matching lines @@
   for (; printed_characters < kLayoutObjectCharacterOffset;
        printed_characters++)
     fputc(' ', stderr);
   fprintf(stderr, "(%d,%d) \"%s\"\n", Start(), Start() + Len(),
           value.Utf8().data());
 }
 
 #endif
 
 }  // namespace blink