Introduce line-gap-override descriptor of @font-face

Bug: 1098355
Change-Id: I30914ddd88b884a6b0241c1184904a67fa7370fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2364001Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799698}

third_party/blink/public/mojom/use_counter/css_property_id.mojom

@@ -718,6 +718,7 @@ enum CSSSampleId {
     kAscentOverride = 672,
     kDescentOverride = 673,
     kLetterSpacingOverride = 674,
+    kLineGapOverride = 675,
     // 1. Add new features above this line (don't change the assigned numbers of
     //    the existing items).
     // 2. Run the src/tools/metrics/histograms/update_use_counter_css.py script

third_party/blink/renderer/core/css/css_properties.json5

@@ -5139,6 +5139,12 @@
       is_property: false,
       runtime_flag: "CSSFontMetricsOverride",
     },
+    {
+      name: "line-gap-override",
+      is_descriptor: true,
+      is_property: false,
+      runtime_flag: "CSSFontMetricsOverride",
+    },
 
     // Shorthands
     {

third_party/blink/renderer/core/css/font_face.cc

@@ -195,6 +195,8 @@ FontFace* FontFace::Create(Document* document,
                                       AtRuleDescriptorID::AscentOverride) &&
       font_face->SetPropertyFromStyle(properties,
                                       AtRuleDescriptorID::DescentOverride) &&
+      font_face->SetPropertyFromStyle(properties,
+                                      AtRuleDescriptorID::LineGapOverride) &&
       font_face->SetPropertyFromStyle(
           properties, AtRuleDescriptorID::LetterSpacingOverride) &&
       font_face->GetFontSelectionCapabilities().IsValid() &&
@@ -365,6 +367,9 @@ bool FontFace::SetPropertyValue(const CSSValue* value,
     case AtRuleDescriptorID::DescentOverride:
       descent_override_ = value;
       break;
+    case AtRuleDescriptorID::LineGapOverride:
+      line_gap_override_ = value;
+      break;
     case AtRuleDescriptorID::LetterSpacingOverride:
       letter_spacing_override_ = value;
       break;
@@ -811,6 +816,7 @@ void FontFace::Trace(Visitor* visitor) const {
   visitor->Trace(display_);
   visitor->Trace(ascent_override_);
   visitor->Trace(descent_override_);
+  visitor->Trace(line_gap_override_);
   visitor->Trace(letter_spacing_override_);
   visitor->Trace(error_);
   visitor->Trace(loaded_property_);
@@ -849,6 +855,10 @@ FontMetricsOverride FontFace::GetFontMetricsOverride() const {
     result.descent_override =
         To<CSSPrimitiveValue>(*descent_override_).GetFloatValue() / 100;
   }
+  if (line_gap_override_) {
+    result.line_gap_override =
+        To<CSSPrimitiveValue>(*line_gap_override_).GetFloatValue() / 100;
+  }
   if (letter_spacing_override_) {
     result.letter_spacing_override =
         To<CSSPrimitiveValue>(*letter_spacing_override_).GetFloatValue();

third_party/blink/renderer/core/css/font_face.h

@@ -135,7 +135,8 @@ class CORE_EXPORT FontFace : public ScriptWrappable,
   bool HasPendingActivity() const final;
 
   bool HasFontMetricsOverride() const {
-    return ascent_override_ || descent_override_ || letter_spacing_override_;
+    return ascent_override_ || descent_override_ || line_gap_override_ ||
+           letter_spacing_override_;
   }
   FontMetricsOverride GetFontMetricsOverride() const;
 
@@ -179,6 +180,7 @@ class CORE_EXPORT FontFace : public ScriptWrappable,
   Member<const CSSValue> display_;
   Member<const CSSValue> ascent_override_;
   Member<const CSSValue> descent_override_;
+  Member<const CSSValue> line_gap_override_;
   Member<const CSSValue> letter_spacing_override_;
   LoadStatusType status_;
   Member<DOMException> error_;

third_party/blink/renderer/core/css/parser/at_rule_descriptor_parser.cc

@@ -266,6 +266,7 @@ CSSValue* AtRuleDescriptorParser::ParseFontFaceDescriptor(
       break;
     case AtRuleDescriptorID::AscentOverride:
     case AtRuleDescriptorID::DescentOverride:
+    case AtRuleDescriptorID::LineGapOverride:
       parsed_value = ConsumeFontMetricOverride(range, context);
       break;
     case AtRuleDescriptorID::LetterSpacingOverride:

third_party/blink/renderer/core/css/parser/at_rule_names.json5

@@ -57,6 +57,9 @@
     {
       name: "letter-spacing-override"
     },
+    {
+      name: "line-gap-override",
+    },
     {
       name: "max-height",
     },

third_party/blink/renderer/platform/fonts/font_metrics_override.h

@@ -12,6 +12,7 @@ namespace blink {
 struct FontMetricsOverride {
   base::Optional<float> ascent_override;
   base::Optional<float> descent_override;
+  base::Optional<float> line_gap_override;
   base::Optional<float> letter_spacing_override;
 };
 

third_party/blink/renderer/platform/fonts/simple_font_data.cc

@@ -126,7 +126,12 @@ void SimpleFontData::PlatformInit(bool subpixel_ascent_descent,
     font_metrics_.SetHasXHeight(false);
   }
 
-  float line_gap = SkScalarToFloat(metrics.fLeading);
+  float line_gap;
+  if (metrics_override.line_gap_override) {
+    line_gap = *metrics_override.line_gap_override * platform_data_.size();
+  } else {
+    line_gap = SkScalarToFloat(metrics.fLeading);
+  }
   font_metrics_.SetLineGap(line_gap);
   font_metrics_.SetLineSpacing(lroundf(ascent) + lroundf(descent) +
                                lroundf(line_gap));

third_party/blink/web_tests/webexposed/css-properties-as-js-properties-expected.txt

@@ -216,6 +216,7 @@ letterSpacing
 letterSpacingOverride
 lightingColor
 lineBreak
+lineGapOverride
 lineHeight
 lineHeightStep
 listStyle

third_party/blink/web_tests/wpt_internal/css/css-fonts/line-gap-override-ref.html

+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Tests the ascent-override and descent-override descriptors of @font-face</title>
+<style>
+.target {
+  position: absolute;
+  font-size: 20px;
+  top: 10px;
+  left: 10px;
+  width: 60px;
+  height: 30px;
+}
+
+.character {
+  display: inline-block;
+  background-color: green;
+  width: 1em;  /* Same as an 'X' in Ahem */
+  height: 1em; /* Same as an 'X' in Ahem */
+  position: absolute;
+}
+
+</style>
+<div class="target">
+  <span class="character" style="top: 0.5em; left: 0"></span>
+  <span class="character" style="top: 2.5em; left: 0"></span>
+</div>

third_party/blink/web_tests/wpt_internal/css/css-fonts/line-gap-override.html

+<!DOCTYPE html>
+<meta charset="utf-8">
+<link rel="author" href="mailto:xiaochengh@chromium.org">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/4792">
+<link rel="match" href="line-gap-override-ref.html">
+<title>Tests the line-gap-override descriptor of @font-face</title>
+<style>
+@font-face {
+  font-family: custom-font;
+  src: local(Ahem), url(/fonts/Ahem.ttf);
+  line-gap-override: 100%;
+}
+
+.target {
+  font: 20px custom-font;
+  color: green;
+  position: absolute;
+  top: 10px;
+  left: 10px;
+}
+</style>
+
+<div class="target">
+  X<br>
+  X
+</div>

tools/metrics/histograms/enums.xml

@@ -45562,6 +45562,7 @@ Called by update_use_counter_css.py.-->
   <int value="672" label="ascent-override"/>
   <int value="673" label="descent-override"/>
   <int value="674" label="letter-spacing-override"/>
+  <int value="675" label="line-gap-override"/>
 </enum>
 
 <enum name="MappedEditingCommands">