Auto merge of #21724 - emilio:gecko-sync, r=emilio

style: Sync changes from mozilla-central

See each individual commit for details.

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---
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17 files changed, 239 insertions, 68 deletions

diff --git a/components/malloc_size_of/Cargo.toml b/components/malloc_size_of/Cargo.toml
index 8a401dd012f..cd05b5d50c0 100644
--- a/components/malloc_size_of/Cargo.toml
+++ b/components/malloc_size_of/Cargo.toml
@@ -15,6 +15,7 @@ servo = [
     "mozjs",
     "serde",
     "serde_bytes",
+    "servo_channel",
     "string_cache",
     "time",
     "url",
@@ -34,7 +35,7 @@ selectors = { path = "../selectors" }
 serde = { version = "1.0.27", optional = true }
 serde_bytes = { version = "0.10", optional = true }
 servo_arc = { path = "../servo_arc" }
-servo_channel = {path = "../channel"}
+servo_channel = { path = "../channel", optional = true }
 smallbitvec = "2.1.0"
 smallvec = "0.6"
 string_cache = { version = "0.7", optional = true }
diff --git a/components/malloc_size_of/lib.rs b/components/malloc_size_of/lib.rs
index d39d162f6b3..72b7f81595a 100644
--- a/components/malloc_size_of/lib.rs
+++ b/components/malloc_size_of/lib.rs
@@ -59,6 +59,7 @@ extern crate serde;
 #[cfg(feature = "servo")]
 extern crate serde_bytes;
 extern crate servo_arc;
+#[cfg(feature = "servo")]
 extern crate servo_channel;
 extern crate smallbitvec;
 extern crate smallvec;
@@ -1024,6 +1025,7 @@ where
 
 // Placeholder for unique case where internals of Sender cannot be measured.
 // malloc size of is 0 macro complains about type supplied!
+#[cfg(feature = "servo")]
 impl<T> MallocSizeOf for servo_channel::Sender<T> {
     fn size_of(&self, _ops: &mut MallocSizeOfOps) -> usize {
         0
diff --git a/components/style/animation.rs b/components/style/animation.rs
index bcb42653cbd..5c0a7103426 100644
--- a/components/style/animation.rs
+++ b/components/style/animation.rs
@@ -4,6 +4,10 @@
 
 //! CSS transitions and animations.
 
+// NOTE(emilio): This code isn't really executed in Gecko, but we don't want to
+// compile it out so that people remember it exists, thus the cfg'd Sender
+// import.
+
 use Atom;
 use bezier::Bezier;
 use context::SharedStyleContext;
@@ -15,8 +19,11 @@ use properties::longhands::animation_direction::computed_value::single_value::T
 use properties::longhands::animation_play_state::computed_value::single_value::T as AnimationPlayState;
 use rule_tree::CascadeLevel;
 use servo_arc::Arc;
+#[cfg(feature = "servo")]
 use servo_channel::Sender;
 use std::fmt;
+#[cfg(feature = "gecko")]
+use std::sync::mpsc::Sender;
 use stylesheets::keyframes_rule::{KeyframesAnimation, KeyframesStep, KeyframesStepValue};
 use timer::Timer;
 use values::computed::Time;
@@ -25,6 +32,7 @@ use values::computed::transform::TimingFunction;
 use values::generics::box_::AnimationIterationCount;
 use values::generics::transform::{StepPosition, TimingFunction as GenericTimingFunction};
 
+
 /// This structure represents a keyframes animation current iteration state.
 ///
 /// If the iteration count is infinite, there's no other state, otherwise we
diff --git a/components/style/gecko/media_features.rs b/components/style/gecko/media_features.rs
index 85f0ba27d43..5c744f24220 100644
--- a/components/style/gecko/media_features.rs
+++ b/components/style/gecko/media_features.rs
@@ -8,6 +8,7 @@ use Atom;
 use app_units::Au;
 use euclid::Size2D;
 use gecko_bindings::bindings;
+use gecko_bindings::structs;
 use media_queries::Device;
 use media_queries::media_feature::{AllowsRanges, ParsingRequirements};
 use media_queries::media_feature::{MediaFeatureDescription, Evaluator};
@@ -300,6 +301,94 @@ fn eval_prefers_reduced_motion(device: &Device, query_value: Option<PrefersReduc
     }
 }
 
+/// https://drafts.csswg.org/mediaqueries-4/#mf-interaction
+bitflags! {
+    struct PointerCapabilities: u8 {
+        const COARSE = structs::PointerCapabilities_Coarse;
+        const FINE = structs::PointerCapabilities_Fine;
+        const HOVER = structs::PointerCapabilities_Hover;
+    }
+}
+
+fn primary_pointer_capabilities(device: &Device) -> PointerCapabilities {
+    PointerCapabilities::from_bits_truncate(
+        unsafe { bindings::Gecko_MediaFeatures_PrimaryPointerCapabilities(device.document()) }
+    )
+}
+
+fn all_pointer_capabilities(device: &Device) -> PointerCapabilities {
+    PointerCapabilities::from_bits_truncate(
+        unsafe { bindings::Gecko_MediaFeatures_AllPointerCapabilities(device.document()) }
+    )
+}
+
+#[derive(Clone, Copy, Debug, FromPrimitive, Parse, ToCss)]
+#[repr(u8)]
+enum Pointer {
+    None,
+    Coarse,
+    Fine,
+}
+
+fn eval_pointer_capabilities(
+    query_value: Option<Pointer>,
+    pointer_capabilities: PointerCapabilities,
+) -> bool {
+    let query_value = match query_value {
+        Some(v) => v,
+        None => return !pointer_capabilities.is_empty(),
+    };
+
+    match query_value {
+        Pointer::None => pointer_capabilities.is_empty(),
+        Pointer::Coarse => pointer_capabilities.intersects(PointerCapabilities::COARSE),
+        Pointer::Fine => pointer_capabilities.intersects(PointerCapabilities::FINE),
+    }
+}
+
+/// https://drafts.csswg.org/mediaqueries-4/#pointer
+fn eval_pointer(device: &Device, query_value: Option<Pointer>) -> bool {
+    eval_pointer_capabilities(query_value, primary_pointer_capabilities(device))
+}
+
+/// https://drafts.csswg.org/mediaqueries-4/#descdef-media-any-pointer
+fn eval_any_pointer(device: &Device, query_value: Option<Pointer>) -> bool {
+    eval_pointer_capabilities(query_value, all_pointer_capabilities(device))
+}
+
+#[derive(Clone, Copy, Debug, FromPrimitive, Parse, ToCss)]
+#[repr(u8)]
+enum Hover {
+    None,
+    Hover,
+}
+
+fn eval_hover_capabilities(
+    query_value: Option<Hover>,
+    pointer_capabilities: PointerCapabilities,
+) -> bool {
+    let can_hover = pointer_capabilities.intersects(PointerCapabilities::HOVER);
+    let query_value = match query_value {
+        Some(v) => v,
+        None => return can_hover,
+    };
+
+    match query_value {
+        Hover::None => !can_hover,
+        Hover::Hover => can_hover,
+    }
+}
+
+/// https://drafts.csswg.org/mediaqueries-4/#hover
+fn eval_hover(device: &Device, query_value: Option<Hover>) -> bool {
+    eval_hover_capabilities(query_value, primary_pointer_capabilities(device))
+}
+
+/// https://drafts.csswg.org/mediaqueries-4/#descdef-media-any-hover
+fn eval_any_hover(device: &Device, query_value: Option<Hover>) -> bool {
+    eval_hover_capabilities(query_value, all_pointer_capabilities(device))
+}
+
 fn eval_moz_is_glyph(
     device: &Device,
     query_value: Option<bool>,
@@ -390,7 +479,7 @@ lazy_static! {
     /// to support new types in these entries and (2) ensuring that either
     /// nsPresContext::MediaFeatureValuesChanged is called when the value that
     /// would be returned by the evaluator function could change.
-    pub static ref MEDIA_FEATURES: [MediaFeatureDescription; 43] = [
+    pub static ref MEDIA_FEATURES: [MediaFeatureDescription; 47] = [
         feature!(
             atom!("width"),
             AllowsRanges::Yes,
@@ -509,6 +598,30 @@ lazy_static! {
             keyword_evaluator!(eval_prefers_reduced_motion, PrefersReducedMotion),
             ParsingRequirements::empty(),
         ),
+        feature!(
+            atom!("pointer"),
+            AllowsRanges::No,
+            keyword_evaluator!(eval_pointer, Pointer),
+            ParsingRequirements::empty(),
+        ),
+        feature!(
+            atom!("any-pointer"),
+            AllowsRanges::No,
+            keyword_evaluator!(eval_any_pointer, Pointer),
+            ParsingRequirements::empty(),
+        ),
+        feature!(
+            atom!("hover"),
+            AllowsRanges::No,
+            keyword_evaluator!(eval_hover, Hover),
+            ParsingRequirements::empty(),
+        ),
+        feature!(
+            atom!("any-hover"),
+            AllowsRanges::No,
+            keyword_evaluator!(eval_any_hover, Hover),
+            ParsingRequirements::empty(),
+        ),
 
         // Internal -moz-is-glyph media feature: applies only inside SVG glyphs.
         // Internal because it is really only useful in the user agent anyway
diff --git a/components/style/gecko/media_queries.rs b/components/style/gecko/media_queries.rs
index ccf5b7a5180..65855b0a8a3 100644
--- a/components/style/gecko/media_queries.rs
+++ b/components/style/gecko/media_queries.rs
@@ -234,10 +234,13 @@ impl Device {
     }
 
     /// Applies text zoom to a font-size or line-height value (see nsStyleFont::ZoomText).
+    #[inline]
     pub fn zoom_text(&self, size: Au) -> Au {
         size.scale_by(self.pres_context().mEffectiveTextZoom)
     }
-    /// Un-apply text zoom (see nsStyleFont::UnzoomText).
+
+    /// Un-apply text zoom.
+    #[inline]
     pub fn unzoom_text(&self, size: Au) -> Au {
         size.scale_by(1. / self.pres_context().mEffectiveTextZoom)
     }
diff --git a/components/style/gecko/url.rs b/components/style/gecko/url.rs
index 8d785cae679..c00fc8c27e3 100644
--- a/components/style/gecko/url.rs
+++ b/components/style/gecko/url.rs
@@ -9,6 +9,7 @@ use gecko_bindings::bindings;
 use gecko_bindings::structs::ServoBundledURI;
 use gecko_bindings::structs::mozilla::css::URLValueData;
 use gecko_bindings::structs::root::{RustString, nsStyleImageRequest};
+use gecko_bindings::structs::root::mozilla::CORSMode;
 use gecko_bindings::structs::root::mozilla::css::{ImageValue, URLValue};
 use gecko_bindings::sugar::refptr::RefPtr;
 use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
@@ -186,15 +187,34 @@ impl SpecifiedImageUrl {
         Self::from_css_url(CssUrl::parse_from_string(url, context))
     }
 
-    fn from_css_url(url: CssUrl) -> Self {
+    fn from_css_url_with_cors(url: CssUrl, cors: CORSMode) -> Self {
         let image_value = unsafe {
-            let ptr = bindings::Gecko_ImageValue_Create(url.for_ffi());
+            let ptr = bindings::Gecko_ImageValue_Create(url.for_ffi(), cors);
             // We do not expect Gecko_ImageValue_Create returns null.
             debug_assert!(!ptr.is_null());
             RefPtr::from_addrefed(ptr)
         };
         Self { url, image_value }
     }
+
+    fn from_css_url(url: CssUrl) -> Self {
+        use gecko_bindings::structs::root::mozilla::CORSMode_CORS_NONE;
+        Self::from_css_url_with_cors(url, CORSMode_CORS_NONE)
+    }
+
+    fn from_css_url_with_cors_anonymous(url: CssUrl) -> Self {
+        use gecko_bindings::structs::root::mozilla::CORSMode_CORS_ANONYMOUS;
+        Self::from_css_url_with_cors(url, CORSMode_CORS_ANONYMOUS)
+    }
+
+    /// Provides an alternate method for parsing that associates the URL
+    /// with anonymous CORS headers.
+    pub fn parse_with_cors_anonymous<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        CssUrl::parse(context, input).map(Self::from_css_url_with_cors_anonymous)
+    }
 }
 
 impl Parse for SpecifiedImageUrl {
diff --git a/components/style/properties/gecko.mako.rs b/components/style/properties/gecko.mako.rs
index 9e9a6835a42..1de99ec16f1 100644
--- a/components/style/properties/gecko.mako.rs
+++ b/components/style/properties/gecko.mako.rs
@@ -2403,7 +2403,8 @@ fn static_assert() {
     /// Calculates the constrained and unconstrained font sizes to be inherited
     /// from the parent.
     ///
-    /// See ComputeScriptLevelSize in Gecko's nsRuleNode.cpp
+    /// This is a port of Gecko's old ComputeScriptLevelSize function:
+    /// https://dxr.mozilla.org/mozilla-central/rev/35fbf14b9/layout/style/nsRuleNode.cpp#3197-3254
     ///
     /// scriptlevel is a property that affects how font-size is inherited. If scriptlevel is
     /// +1, for example, it will inherit as the script size multiplier times
@@ -2511,17 +2512,18 @@ fn static_assert() {
             = self.calculate_script_level_size(parent, device);
         if adjusted_size.0 != parent.gecko.mSize ||
            adjusted_unconstrained_size.0 != parent.gecko.mScriptUnconstrainedSize {
-            // This is incorrect. When there is both a keyword size being inherited
-            // and a scriptlevel change, we must handle the keyword size the same
-            // way we handle em units. This complicates things because we now have
-            // to keep track of the adjusted and unadjusted ratios in the kw font size.
-            // This only affects the use case of a generic font being used in MathML.
+            // FIXME(Manishearth): This is incorrect. When there is both a
+            // keyword size being inherited and a scriptlevel change, we must
+            // handle the keyword size the same way we handle em units. This
+            // complicates things because we now have to keep track of the
+            // adjusted and unadjusted ratios in the kw font size. This only
+            // affects the use case of a generic font being used in MathML.
             //
-            // If we were to fix this I would prefer doing it by removing the
-            // ruletree walk on the Gecko side in nsRuleNode::SetGenericFont
-            // and instead using extra bookkeeping in the mSize and mScriptUnconstrainedSize
-            // values, and reusing those instead of font_size_keyword.
-
+            // If we were to fix this I would prefer doing it not doing
+            // something like the ruletree walk that Gecko used to do in
+            // nsRuleNode::SetGenericFont and instead using extra bookkeeping in
+            // the mSize and mScriptUnconstrainedSize values, and reusing those
+            // instead of font_size_keyword.
 
             // In the case that MathML has given us an adjusted size, apply it.
             // Keep track of the unconstrained adjusted size.
diff --git a/components/style/rule_tree/mod.rs b/components/style/rule_tree/mod.rs
index d4fb0d00925..26c089eb4ad 100644
--- a/components/style/rule_tree/mod.rs
+++ b/components/style/rule_tree/mod.rs
@@ -1200,9 +1200,6 @@ impl StrongRuleNode {
         }
     }
 
-    /// Implementation of `nsRuleNode::HasAuthorSpecifiedRules` for Servo rule
-    /// nodes.
-    ///
     /// Returns true if any properties specified by `rule_type_mask` was set by
     /// an author rule.
     #[cfg(feature = "gecko")]
diff --git a/components/style/servo/url.rs b/components/style/servo/url.rs
index b4821f21e66..91521c8a120 100644
--- a/components/style/servo/url.rs
+++ b/components/style/servo/url.rs
@@ -98,6 +98,18 @@ impl CssUrl {
             resolved: ServoUrl::parse(url).ok(),
         }
     }
+
+    /// Parses a URL request and records that the corresponding request needs to
+    /// be CORS-enabled.
+    ///
+    /// This is only for shape images and masks in Gecko, thus unimplemented for
+    /// now so somebody notices when trying to do so.
+    pub fn parse_with_cors_anonymous<'i, 't>(
+        _context: &ParserContext,
+        _input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        unimplemented!("Need to record somewhere that the request needs to be CORS-enabled")
+    }
 }
 
 impl Parse for CssUrl {
diff --git a/components/style/values/animated/effects.rs b/components/style/values/animated/effects.rs
index a201ec0549e..a86396506df 100644
--- a/components/style/values/animated/effects.rs
+++ b/components/style/values/animated/effects.rs
@@ -11,7 +11,6 @@ use values::computed::{Angle, Number};
 use values::computed::length::Length;
 #[cfg(feature = "gecko")]
 use values::computed::url::ComputedUrl;
-use values::distance::{ComputeSquaredDistance, SquaredDistance};
 use values::generics::effects::BoxShadow as GenericBoxShadow;
 use values::generics::effects::Filter as GenericFilter;
 use values::generics::effects::SimpleShadow as GenericSimpleShadow;
@@ -29,14 +28,3 @@ pub type Filter = GenericFilter<Angle, Number, Length, Impossible, Impossible>;
 
 /// An animated value for the `drop-shadow()` filter.
 pub type SimpleShadow = GenericSimpleShadow<Color, Length, Length>;
-
-impl ComputeSquaredDistance for BoxShadow {
-    #[inline]
-    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
-        if self.inset != other.inset {
-            return Err(());
-        }
-        Ok(self.base.compute_squared_distance(&other.base)? +
-            self.spread.compute_squared_distance(&other.spread)?)
-    }
-}
diff --git a/components/style/values/generics/effects.rs b/components/style/values/generics/effects.rs
index 7c7e3f4bef3..7b8e64f90bf 100644
--- a/components/style/values/generics/effects.rs
+++ b/components/style/values/generics/effects.rs
@@ -8,6 +8,7 @@
 #[derive(
     Animate,
     Clone,
+    ComputeSquaredDistance,
     Debug,
     MallocSizeOf,
     PartialEq,
diff --git a/components/style/values/generics/font.rs b/components/style/values/generics/font.rs
index 03a76c5a32d..fa1e160273c 100644
--- a/components/style/values/generics/font.rs
+++ b/components/style/values/generics/font.rs
@@ -13,7 +13,6 @@ use std::fmt::{self, Write};
 use std::io::Cursor;
 use style_traits::{CssWriter, KeywordsCollectFn, ParseError};
 use style_traits::{SpecifiedValueInfo, StyleParseErrorKind, ToCss};
-use values::distance::{ComputeSquaredDistance, SquaredDistance};
 
 /// https://drafts.csswg.org/css-fonts-4/#feature-tag-value
 #[derive(Clone, Debug, Eq, MallocSizeOf, PartialEq, SpecifiedValueInfo, ToComputedValue)]
@@ -47,7 +46,16 @@ where
 ///
 /// https://drafts.csswg.org/css-fonts-4/#font-variation-settings-def
 #[derive(
-    Animate, Clone, Debug, Eq, MallocSizeOf, PartialEq, SpecifiedValueInfo, ToComputedValue, ToCss,
+    Animate,
+    Clone,
+    ComputeSquaredDistance,
+    Debug,
+    Eq,
+    MallocSizeOf,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToComputedValue,
+    ToCss,
 )]
 pub struct VariationValue<Number> {
     /// A four-character tag, packed into a u32 (one byte per character).
@@ -57,19 +65,6 @@ pub struct VariationValue<Number> {
     pub value: Number,
 }
 
-impl<Number> ComputeSquaredDistance for VariationValue<Number>
-where
-    Number: ComputeSquaredDistance,
-{
-    #[inline]
-    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
-        if self.tag != other.tag {
-            return Err(());
-        }
-        self.value.compute_squared_distance(&other.value)
-    }
-}
-
 /// A value both for font-variation-settings and font-feature-settings.
 #[css(comma)]
 #[derive(Clone, Debug, Eq, MallocSizeOf, PartialEq, SpecifiedValueInfo, ToComputedValue, ToCss)]
diff --git a/components/style/values/specified/basic_shape.rs b/components/style/values/specified/basic_shape.rs
index cdafbf14b37..52343621b5e 100644
--- a/components/style/values/specified/basic_shape.rs
+++ b/components/style/values/specified/basic_shape.rs
@@ -71,7 +71,12 @@ impl Parse for ClippingShape {
                 return Ok(ShapeSource::Path(p));
             }
         }
-        Self::parse_internal(context, input)
+
+        if let Ok(url) = input.try(|i| SpecifiedUrl::parse(context, i)) {
+            return Ok(ShapeSource::ImageOrUrl(url));
+        }
+
+        Self::parse_common(context, input)
     }
 }
 
@@ -81,17 +86,20 @@ impl Parse for FloatAreaShape {
         context: &ParserContext,
         input: &mut Parser<'i, 't>,
     ) -> Result<Self, ParseError<'i>> {
-        Self::parse_internal(context, input)
+        if let Ok(image) = input.try(|i| Image::parse_with_cors_anonymous(context, i)) {
+            return Ok(ShapeSource::ImageOrUrl(image));
+        }
+
+        Self::parse_common(context, input)
     }
 }
 
 impl<ReferenceBox, ImageOrUrl> ShapeSource<BasicShape, ReferenceBox, ImageOrUrl>
 where
     ReferenceBox: Parse,
-    ImageOrUrl: Parse,
 {
     /// The internal parser for ShapeSource.
-    fn parse_internal<'i, 't>(
+    fn parse_common<'i, 't>(
         context: &ParserContext,
         input: &mut Parser<'i, 't>,
     ) -> Result<Self, ParseError<'i>> {
@@ -99,10 +107,6 @@ where
             return Ok(ShapeSource::None);
         }
 
-        if let Ok(image_or_url) = input.try(|i| ImageOrUrl::parse(context, i)) {
-            return Ok(ShapeSource::ImageOrUrl(image_or_url));
-        }
-
         fn parse_component<U: Parse>(
             context: &ParserContext,
             input: &mut Parser,
diff --git a/components/style/values/specified/font.rs b/components/style/values/specified/font.rs
index d9cf921bca5..5613ae603d2 100644
--- a/components/style/values/specified/font.rs
+++ b/components/style/values/specified/font.rs
@@ -746,7 +746,12 @@ impl ToComputedValue for KeywordSize {
     fn to_computed_value(&self, cx: &Context) -> NonNegativeLength {
         use context::QuirksMode;
         use values::specified::length::au_to_int_px;
-        // Data from nsRuleNode.cpp in Gecko
+
+        // The tables in this function are originally from
+        // nsRuleNode::CalcFontPointSize in Gecko:
+        //
+        // https://dxr.mozilla.org/mozilla-central/rev/35fbf14b9/layout/style/nsRuleNode.cpp#3262-3336
+
         // Mapping from base size and HTML size to pixels
         // The first index is (base_size - 9), the second is the
         // HTML size. "0" is CSS keyword xx-small, not HTML size 0,
@@ -765,9 +770,6 @@ impl ToComputedValue for KeywordSize {
             [9, 10, 13, 16, 18, 24, 32, 48],
         ];
 
-        // Data from nsRuleNode.cpp in Gecko
-        // (https://dxr.mozilla.org/mozilla-central/rev/35fbf14b9/layout/style/nsRuleNode.cpp#3303)
-        //
         // This table gives us compatibility with WinNav4 for the default fonts only.
         // In WinNav4, the default fonts were:
         //
diff --git a/components/style/values/specified/image.rs b/components/style/values/specified/image.rs
index 1c0a613e488..1b080816e83 100644
--- a/components/style/values/specified/image.rs
+++ b/components/style/values/specified/image.rs
@@ -166,6 +166,21 @@ impl Image {
             ref t => Err(location.new_unexpected_token_error(t.clone())),
         })
     }
+
+    /// Provides an alternate method for parsing that associates the URL with
+    /// anonymous CORS headers.
+    ///
+    /// FIXME(emilio): It'd be nicer for this to pass a `CorsMode` parameter to
+    /// a shared function instead.
+    pub fn parse_with_cors_anonymous<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Image, ParseError<'i>> {
+        if let Ok(url) = input.try(|input| SpecifiedImageUrl::parse_with_cors_anonymous(context, input)) {
+            return Ok(generic::Image::Url(url));
+        }
+        Self::parse(context, input)
+    }
 }
 
 impl Parse for Gradient {
diff --git a/components/style/values/specified/svg_path.rs b/components/style/values/specified/svg_path.rs
index f0973b545a5..5d397558d25 100644
--- a/components/style/values/specified/svg_path.rs
+++ b/components/style/values/specified/svg_path.rs
@@ -538,13 +538,6 @@ impl ToCss for ArcFlag {
     }
 }
 
-impl ComputeSquaredDistance for ArcFlag {
-    #[inline]
-    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
-        (self.0 as i32).compute_squared_distance(&(other.0 as i32))
-    }
-}
-
 /// SVG Path parser.
 struct PathParser<'a> {
     chars: Peekable<Cloned<slice::Iter<'a, u8>>>,
diff --git a/components/style_derive/compute_squared_distance.rs b/components/style_derive/compute_squared_distance.rs
index 5b414206cda..f7e50c6bb00 100644
--- a/components/style_derive/compute_squared_distance.rs
+++ b/components/style_derive/compute_squared_distance.rs
@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use animate::{AnimationInputAttrs, AnimationVariantAttrs};
+use animate::{AnimationInputAttrs, AnimationVariantAttrs, AnimationFieldAttrs};
 use cg;
 use quote::Tokens;
 use syn::{DeriveInput, Path};
@@ -47,8 +47,23 @@ pub fn derive(mut input: DeriveInput) -> Tokens {
                             parse_quote!(#ty: ::values::distance::ComputeSquaredDistance),
                         );
                     }
-                    quote! {
-                        ::values::distance::ComputeSquaredDistance::compute_squared_distance(#this, #other)?
+
+                    let animation_field_attrs =
+                        cg::parse_field_attrs::<AnimationFieldAttrs>(&this.ast());
+
+                    if animation_field_attrs.constant {
+                        quote! {
+                            {
+                                if #this != #other {
+                                    return Err(());
+                                }
+                                ::values::distance::SquaredDistance::from_sqrt(0.)
+                            }
+                        }
+                    } else {
+                        quote! {
+                            ::values::distance::ComputeSquaredDistance::compute_squared_distance(#this, #other)?
+                        }
                     }
                 }), quote!(+));
                 sum