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

third_party/WebKit/Source/core/layout/line/InlineBox.h

 /*
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 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.
  *
  * This library is distributed in the hope that it will be useful,
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 namespace blink {
 
 class HitTestRequest;
 class HitTestResult;
 class InlineFlowBox;
 class LayoutObject;
 class RootInlineBox;
 
 enum MarkLineBoxes { kMarkLineBoxesDirty, kDontMarkLineBoxes };
 
+enum class LineVerticalPositionType {
+  // TextTop and TextBottom are the top/bottom of the content area.
+  // This is where 'vertical-align: text-top/text-bottom' aligns to.
+  // This is explicitly undefined in CSS2.
+  // https://drafts.csswg.org/css2/visudet.html#inline-non-replaced
+  TextTop,
+  TextBottom,
+  // Em height as being discussed in Font Metrics API.
+  // https://drafts.css-houdini.org/font-metrics-api-1/#fontmetrics
+  TopOfEmHeight,
+  BottomOfEmHeight
+};
+
+// Returns whether the position type is CSS "line-over"; i.e., ascender side
+// or "top" side of a line box.
+// https://drafts.csswg.org/css-writing-modes-3/#line-over
+static inline bool IsLineOverSide(LineVerticalPositionType type) {
+  return type == LineVerticalPositionType::TextTop ||
+         type == LineVerticalPositionType::TopOfEmHeight;
+}
+
 // InlineBox represents a rectangle that occurs on a line.  It corresponds to
 // some LayoutObject (i.e., it represents a portion of that LayoutObject).
 class CORE_EXPORT InlineBox : public DisplayItemClient {
   WTF_MAKE_NONCOPYABLE(InlineBox);
 
  public:
   InlineBox(LineLayoutItem obj)
       : next_(nullptr),
         prev_(nullptr),
         parent_(nullptr),
@@ skipping 493 matching lines @@
 
 }  // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showTree(const blink::InlineBox*);
 void showLineTree(const blink::InlineBox*);
 #endif
 
 #endif  // InlineBox_h