Bug 1387594 - Add a chrome-only CSS property called -moz-font-smoothing-background-color. r=dbaron

This property accepts a color. It's inherited and defaults to transparent.
Its value is respected on macOS when rendering text into transparent pixels.
This property should be used for text that is placed on top of "vibrant"
-moz-appearances, in order to achieve high quality text rendering for such text.

In most cases, the property should be set to a named system color; an upcoming
patch in this patch series will add one such color for each vibrant
-moz-appearance value.
However, in some cases it can also be useful to use a custom color: If text
is rendered into an intermediate surface, for example because a mask is applied
to it, and the background color behind that intermediate surface is known, then
this property can be set to that background color in order to achieve subpixel
AA for the text inside the mask effect. In that case, the font smoothing
background color is respected because text is rendered into transparent pixels
*inside the intermediate surface*. At the moment, the only example of that use
case is the text of the active tab in the state where the text is overflowing.

MozReview-Commit-ID: D98qQnxoFaq

devtools/shared/css/generated/properties-db.js

@@ -3013,6 +3013,7 @@ exports.CSS_PROPERTIES = {
       "font-language-override",
       "font-size",
       "font-size-adjust",
+      "-moz-font-smoothing-background-color",
       "font-stretch",
       "font-style",
       "font-synthesis",

layout/style/nsCSSPropList.h

@@ -1951,6 +1951,20 @@ CSS_PROP_FONT(
     nullptr,
     offsetof(nsStyleFont, mFont.sizeAdjust),
     eStyleAnimType_float)
+#ifndef CSS_PROP_LIST_EXCLUDE_INTERNAL
+CSS_PROP_USERINTERFACE(
+    -moz-font-smoothing-background-color,
+    _moz_font_smoothing_background_color,
+    CSS_PROP_DOMPROP_PREFIXED(FontSmoothingBackgroundColor),
+    CSS_PROPERTY_INTERNAL |
+        CSS_PROPERTY_PARSE_VALUE |
+        CSS_PROPERTY_ENABLED_IN_UA_SHEETS_AND_CHROME,
+    "",
+    VARIANT_HC,
+    nullptr,
+    offsetof(nsStyleUserInterface, mFontSmoothingBackgroundColor),
+    eStyleAnimType_Color)
+#endif // CSS_PROP_LIST_EXCLUDE_INTERNAL
 CSS_PROP_FONT(
     font-stretch,
     font_stretch,

layout/style/nsComputedDOMStyle.cpp

@@ -1766,6 +1766,14 @@ nsComputedDOMStyle::DoGetOsxFontSmoothing()
   return val.forget();
 }
 
+already_AddRefed<CSSValue>
+nsComputedDOMStyle::DoGetFontSmoothingBackgroundColor()
+{
+  RefPtr<nsROCSSPrimitiveValue> val = new nsROCSSPrimitiveValue;
+  SetToRGBAColor(val, StyleUserInterface()->mFontSmoothingBackgroundColor);
+  return val.forget();
+}
+
 already_AddRefed<CSSValue>
 nsComputedDOMStyle::DoGetFontStretch()
 {

layout/style/nsComputedDOMStyle.h

@@ -296,6 +296,7 @@ private:
   already_AddRefed<CSSValue> DoGetFontSize();
   already_AddRefed<CSSValue> DoGetFontSizeAdjust();
   already_AddRefed<CSSValue> DoGetOsxFontSmoothing();
+  already_AddRefed<CSSValue> DoGetFontSmoothingBackgroundColor();
   already_AddRefed<CSSValue> DoGetFontStretch();
   already_AddRefed<CSSValue> DoGetFontStyle();
   already_AddRefed<CSSValue> DoGetFontSynthesis();

layout/style/nsComputedDOMStylePropertyList.h

@@ -288,6 +288,7 @@ COMPUTED_STYLE_PROP(_moz_box_orient,               BoxOrient)
 COMPUTED_STYLE_PROP(_moz_box_pack,                 BoxPack)
 COMPUTED_STYLE_PROP(_moz_context_properties,       ContextProperties)
 COMPUTED_STYLE_PROP(_moz_float_edge,               FloatEdge)
+COMPUTED_STYLE_PROP(_moz_font_smoothing_background_color, FontSmoothingBackgroundColor)
 COMPUTED_STYLE_PROP(_moz_force_broken_image_icon,  ForceBrokenImageIcon)
 COMPUTED_STYLE_PROP(_moz_image_region,             ImageRegion)
 COMPUTED_STYLE_PROP(_moz_orient,                   Orient)

layout/style/nsRuleNode.cpp

@@ -5326,6 +5326,18 @@ nsRuleNode::ComputeUserInterfaceData(void* aStartStruct,
                                  mPresContext,
                                  ui->mCaretColor, conditions);
 
+  // -moz-font-smoothing-background-color:
+  const nsCSSValue* fsbColorValue =
+    aRuleData->ValueForFontSmoothingBackgroundColor();
+  if (eCSSUnit_Initial == fsbColorValue->GetUnit() ||
+      eCSSUnit_Unset == fsbColorValue->GetUnit()) {
+    ui->mFontSmoothingBackgroundColor = NS_RGBA(0, 0, 0, 0);
+  } else {
+    SetColor(*fsbColorValue, parentUI->mFontSmoothingBackgroundColor,
+             mPresContext, aContext, ui->mFontSmoothingBackgroundColor,
+             conditions);
+  }
+
   COMPUTE_END_INHERITED(UserInterface, ui)
 }
 

layout/style/nsStyleStruct.cpp

@@ -4325,6 +4325,7 @@ nsStyleUserInterface::nsStyleUserInterface(const nsPresContext* aContext)
   , mPointerEvents(NS_STYLE_POINTER_EVENTS_AUTO)
   , mCursor(NS_STYLE_CURSOR_AUTO)
   , mCaretColor(StyleComplexColor::Auto())
+  , mFontSmoothingBackgroundColor(NS_RGBA(0, 0, 0, 0))
 {
   MOZ_COUNT_CTOR(nsStyleUserInterface);
 }
@@ -4337,6 +4338,7 @@ nsStyleUserInterface::nsStyleUserInterface(const nsStyleUserInterface& aSource)
   , mCursor(aSource.mCursor)
   , mCursorImages(aSource.mCursorImages)
   , mCaretColor(aSource.mCaretColor)
+  , mFontSmoothingBackgroundColor(aSource.mFontSmoothingBackgroundColor)
 {
   MOZ_COUNT_CTOR(nsStyleUserInterface);
 }
@@ -4398,7 +4400,8 @@ nsStyleUserInterface::CalcDifference(const nsStyleUserInterface& aNewData) const
     hint |= nsChangeHint_NeutralChange;
   }
 
-  if (mCaretColor != aNewData.mCaretColor) {
+  if (mCaretColor != aNewData.mCaretColor ||
+      mFontSmoothingBackgroundColor != aNewData.mFontSmoothingBackgroundColor) {
     hint |= nsChangeHint_RepaintFrame;
   }
 

layout/style/nsStyleStruct.h

@@ -3319,6 +3319,7 @@ struct MOZ_NEEDS_MEMMOVABLE_MEMBERS nsStyleUserInterface
   uint8_t mCursor;                            // [inherited] See nsStyleConsts.h
   nsTArray<nsCursorImage> mCursorImages;      // [inherited] images and coords
   mozilla::StyleComplexColor mCaretColor;     // [inherited]
+  nscolor                   mFontSmoothingBackgroundColor; // [inherited]
 
   inline uint8_t GetEffectivePointerEvents(nsIFrame* aFrame) const;
 };

layout/style/test/ListCSSProperties.cpp

@@ -115,6 +115,7 @@ const char *gInaccessibleProperties[] = {
     "-moz-math-display", // parsed by UA sheets only
     "-moz-top-layer", // parsed by UA sheets only
     "-moz-min-font-size-ratio", // parsed by UA sheets only
+    "-moz-font-smoothing-background-color", // chrome-only internal properties
     "-moz-window-opacity", // chrome-only internal properties
     "-moz-window-transform", // chrome-only internal properties
     "-moz-window-transform-origin", // chrome-only internal properties

layout/style/test/property_database.js

@@ -8291,4 +8291,13 @@ if (false) {
     other_values: [ "fill", "stroke", "fill, stroke", "fill, stroke, fill", "fill, foo", "foo" ],
     invalid_values: [ "default", "fill, auto", "all, stroke", "none, fill", "fill, none", "fill, default", "2px" ]
   };
+
+  gCSSProperties["-moz-font-smoothing-background-color"] = {
+    // domProp: "MozFontSmoothingBackgroundColor",
+    inherited: true,
+    type: CSS_TYPE_LONGHAND,
+    initial_values: [ "transparent" ],
+    other_values: [ "green", "#fc3" ],
+    invalid_values: [ "000000", "ff00ff" ]
+  };
 }