Bug 1810815 - Properly account for cases where CSS units ch or ic are based on a different font than the first, due to character coverage. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D167070

gfx/thebes/gfxTextRun.cpp

@@ -2284,7 +2284,7 @@ already_AddRefed<gfxFont> gfxFontGroup::GetDefaultFont() {
 }
 
 already_AddRefed<gfxFont> gfxFontGroup::GetFirstValidFont(
-    uint32_t aCh, StyleGenericFontFamily* aGeneric) {
+    uint32_t aCh, StyleGenericFontFamily* aGeneric, bool* aIsFirst) {
   // Ensure cached font instances are valid.
   CheckForUpdatedPlatformList();
 
@@ -2319,6 +2319,9 @@ already_AddRefed<gfxFont> gfxFontGroup::GetFirstValidFont(
       if (aGeneric) {
         *aGeneric = ff.Generic();
       }
+      if (aIsFirst) {
+        *aIsFirst = (i == 0);
+      }
       return font.forget();
     }
 
@@ -2349,12 +2352,18 @@ already_AddRefed<gfxFont> gfxFontGroup::GetFirstValidFont(
       if (aGeneric) {
         *aGeneric = ff.Generic();
       }
+      if (aIsFirst) {
+        *aIsFirst = (i == 0);
+      }
       return font.forget();
     }
   }
   if (aGeneric) {
     *aGeneric = StyleGenericFontFamily::None;
   }
+  if (aIsFirst) {
+    *aIsFirst = false;
+  }
   return GetDefaultFont();
 }
 
@@ -3831,6 +3840,35 @@ already_AddRefed<gfxFont> gfxFontGroup::WhichSystemFontSupportsChar(
       &visibility);
 }
 
+gfxFont::Metrics gfxFontGroup::GetMetricsForCSSUnits(
+    gfxFont::Orientation aOrientation) {
+  bool isFirst;
+  RefPtr<gfxFont> font = GetFirstValidFont(0x20, nullptr, &isFirst);
+  auto metrics = font->GetMetrics(aOrientation);
+
+  // If the font we used to get metrics was not the first in the list,
+  // or if it doesn't support the ZERO character, check for the font that
+  // does support ZERO and use its metrics for the 'ch' unit.
+  if (!isFirst || !font->HasCharacter('0')) {
+    RefPtr<gfxFont> zeroFont = GetFirstValidFont('0');
+    if (zeroFont != font) {
+      const auto& zeroMetrics = zeroFont->GetMetrics(aOrientation);
+      metrics.zeroWidth = zeroMetrics.zeroWidth;
+    }
+  }
+
+  // Likewise for the WATER ideograph character used as the basis for 'ic'.
+  if (!isFirst || !font->HasCharacter(0x6C34)) {
+    RefPtr<gfxFont> icFont = GetFirstValidFont(0x6C34);
+    if (icFont != font) {
+      const auto& icMetrics = icFont->GetMetrics(aOrientation);
+      metrics.ideographicWidth = icMetrics.ideographicWidth;
+    }
+  }
+
+  return metrics;
+}
+
 void gfxMissingFontRecorder::Flush() {
   static bool mNotifiedFontsInitialized = false;
   static uint32_t mNotifiedFonts[gfxMissingFontRecorder::kNumScriptBitsWords];

gfx/thebes/gfxTextRun.h

@@ -947,8 +947,10 @@ class gfxFontGroup final : public gfxTextRunFactory {
   // Initiates userfont loads if userfont not loaded.
   // aGeneric: if non-null, returns the CSS generic type that was mapped to
   //           this font
+  // aIsFirst: if non-null, returns whether the font was first in the list
   already_AddRefed<gfxFont> GetFirstValidFont(
-      uint32_t aCh = 0x20, mozilla::StyleGenericFontFamily* aGeneric = nullptr);
+      uint32_t aCh = 0x20, mozilla::StyleGenericFontFamily* aGeneric = nullptr,
+      bool* aIsFirst = nullptr);
 
   // Returns the first font in the font-group that has an OpenType MATH table,
   // or null if no such font is available. The GetMathConstant methods may be
@@ -1104,6 +1106,14 @@ class gfxFontGroup final : public gfxTextRunFactory {
 
   nsAtom* Language() const { return mLanguage.get(); }
 
+  // Get font metrics to be used as the basis for CSS font-relative units.
+  // Note that these may be a "composite" of metrics from multiple fonts,
+  // because the 'ch' and 'ic' units depend on the font that would be used
+  // to render specific characters, not simply the "first available" font.
+  // https://drafts.csswg.org/css-values-4/#ch
+  // https://drafts.csswg.org/css-values-4/#ic
+  gfxFont::Metrics GetMetricsForCSSUnits(gfxFont::Orientation aOrientation);
+
  protected:
   friend class mozilla::PostTraversalTask;
 

layout/style/GeckoBindings.cpp

@@ -1431,17 +1431,20 @@ GeckoFontMetrics Gecko_GetFontMetrics(const nsPresContext* aPresContext,
   nsPresContext* presContext = const_cast<nsPresContext*>(aPresContext);
   RefPtr<nsFontMetrics> fm = nsLayoutUtils::GetMetricsFor(
       presContext, aIsVertical, aFont, aFontSize, aUseUserFontSet);
-  RefPtr<gfxFont> font = fm->GetThebesFontGroup()->GetFirstValidFont();
-  const auto& metrics = font->GetMetrics(fm->Orientation());
+  auto* fontGroup = fm->GetThebesFontGroup();
+  auto metrics = fontGroup->GetMetricsForCSSUnits(fm->Orientation());
 
   float scriptPercentScaleDown = 0;
   float scriptScriptPercentScaleDown = 0;
-  if (aRetrieveMathScales && font->TryGetMathTable()) {
-    scriptPercentScaleDown = static_cast<float>(
-        font->MathTable()->Constant(gfxMathTable::ScriptPercentScaleDown));
-    scriptScriptPercentScaleDown =
-        static_cast<float>(font->MathTable()->Constant(
-            gfxMathTable::ScriptScriptPercentScaleDown));
+  if (aRetrieveMathScales) {
+    RefPtr<gfxFont> font = fontGroup->GetFirstValidFont();
+    if (font->TryGetMathTable()) {
+      scriptPercentScaleDown = static_cast<float>(
+          font->MathTable()->Constant(gfxMathTable::ScriptPercentScaleDown));
+      scriptScriptPercentScaleDown =
+          static_cast<float>(font->MathTable()->Constant(
+              gfxMathTable::ScriptScriptPercentScaleDown));
+    }
   }
 
   int32_t d2a = aPresContext->AppUnitsPerDevPixel();