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

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

Index: third_party/WebKit/Source/platform/fonts/SimpleFontData.cpp
diff --git a/third_party/WebKit/Source/platform/fonts/SimpleFontData.cpp b/third_party/WebKit/Source/platform/fonts/SimpleFontData.cpp
index d3224097198037903690caeb0d420a23b89898bf..db238b4fcb8cde4b40212ce38746767bd1139039 100644
--- a/third_party/WebKit/Source/platform/fonts/SimpleFontData.cpp
+++ b/third_party/WebKit/Source/platform/fonts/SimpleFontData.cpp
@@ -39,6 +39,7 @@
 #include "platform/fonts/VDMXParser.h"
 #include "platform/fonts/skia/SkiaTextMetrics.h"
 #include "platform/geometry/FloatRect.h"
+#include "platform/wtf/ByteOrder.h"
 #include "platform/wtf/MathExtras.h"
 #include "platform/wtf/PtrUtil.h"
 #include "platform/wtf/allocator/Partitions.h"
@@ -379,6 +380,84 @@ PassRefPtr<SimpleFontData> SimpleFontData::CreateScaledFontData(
       IsCustomFont() ? CustomFontData::Create() : nullptr);
 }
 
+// Internal leadings can be distributed to ascent and descent.
+// -------------------------------------------
+//           | - Internal Leading (in ascent)
+//           |--------------------------------
+//  Ascent - |              |
+//           |              |
+//           |              | - Em height
+// ----------|--------------|
+//           |              |
+// Descent - |--------------------------------
+//           | - Internal Leading (in descent)

[eae, 2017/05/01 16:21:48]

This illustration really helps, thank you!

+// -------------------------------------------
+// How Gecko computes "em height":
+// https://github.com/whatwg/html/issues/2470#issuecomment-291425136
+LayoutUnit SimpleFontData::EmHeightAscent(FontBaseline baseline_type) const {
+  if (baseline_type == kAlphabeticBaseline) {
+    if (!em_height_ascent_)
+      ComputeEmHeightMetrics();
+    return em_height_ascent_;
+  }
+  LayoutUnit em_height = LayoutUnit::FromFloatRound(PlatformData().size());
+  return em_height - em_height / 2;
+}
+
+LayoutUnit SimpleFontData::EmHeightDescent(FontBaseline baseline_type) const {
+  if (baseline_type == kAlphabeticBaseline) {
+    if (!em_height_descent_)
+      ComputeEmHeightMetrics();
+    return em_height_descent_;
+  }
+  LayoutUnit em_height = LayoutUnit::FromFloatRound(PlatformData().size());
+  return em_height / 2;
+}
+
+static std::pair<int16_t, int16_t> TypoAscenderAndDescender(
+    SkTypeface* typeface) {
+  // TODO(kojii): This should move to Skia once finalized. We can then move
+  // EmHeightAscender/Descender to FontMetrics.
+  int16_t buffer[2];
+  size_t size = typeface->getTableData(SkSetFourByteTag('O', 'S', '/', '2'), 68,
+                                       sizeof(buffer), buffer);
+  if (size == sizeof(buffer)) {
+    return std::make_pair(static_cast<int16_t>(ntohs(buffer[0])),
+                          -static_cast<int16_t>(ntohs(buffer[1])));
+  }
+  return std::make_pair(0, 0);
+}
+
+void SimpleFontData::ComputeEmHeightMetrics() const {
+  // Compute em height metrics from OS/2 sTypoAscender and sTypoDescender.
+  SkTypeface* typeface = platform_data_.Typeface();
+  int16_t typo_ascender, typo_descender;
+  std::tie(typo_ascender, typo_descender) = TypoAscenderAndDescender(typeface);
+  if (typo_ascender > 0 &&
+      NormalizeEmHeightMetrics(typo_ascender, typo_ascender + typo_descender)) {
+    return;
+  }
+
+  // As the last resort, compute em height metrics from our ascent/descent.
+  const FontMetrics& font_metrics = GetFontMetrics();
+  if (NormalizeEmHeightMetrics(font_metrics.FloatAscent(),
+                               font_metrics.FloatHeight())) {
+    return;
+  }
+  NOTREACHED();
+}
+
+bool SimpleFontData::NormalizeEmHeightMetrics(float ascent,

[eae, 2017/05/01 16:21:48]

A comment explaining why normalization might be needed would be helpful. Either
here or up at line 396.

[kojii, 2017/05/02 22:56:52]

Done.

+                                              float height) const {
+  if (height <= 0 || ascent < 0 || ascent > height)
+    return false;
+  float em_height = PlatformData().size();
+  em_height_ascent_ = LayoutUnit::FromFloatRound(ascent * em_height / height);
+  em_height_descent_ =
+      LayoutUnit::FromFloatRound(em_height) - em_height_ascent_;
+  return true;
+}
+
 FloatRect SimpleFontData::PlatformBoundsForGlyph(Glyph glyph) const {
   if (!platform_data_.size())
     return FloatRect();