style: Support offset-anchor.

Differential Revision: https://phabricator.services.mozilla.com/D39432

components/style/properties/longhands/box.mako.rs

@@ -421,6 +421,19 @@ ${helpers.predefined_type(
     servo_restyle_damage="reflow_out_of_flow"
 )}
 
+// Motion Path Module Level 1
+${helpers.predefined_type(
+    "offset-anchor",
+    "PositionOrAuto",
+    "computed::PositionOrAuto::auto()",
+    engines="gecko",
+    animation_value_type="none",
+    gecko_pref="layout.css.motion-path.enabled",
+    spec="https://drafts.fxtf.org/motion-1/#offset-anchor-property",
+    servo_restyle_damage="reflow_out_of_flow",
+    boxed=True
+)}
+
 // CSSOM View Module
 // https://www.w3.org/TR/cssom-view-1/
 ${helpers.single_keyword(

components/style/values/computed/mod.rs

@@ -68,7 +68,7 @@ pub use self::list::Quotes;
 pub use self::motion::{OffsetPath, OffsetRotate};
 pub use self::outline::OutlineStyle;
 pub use self::percentage::{NonNegativePercentage, Percentage};
-pub use self::position::{GridAutoFlow, GridTemplateAreas, Position, ZIndex};
+pub use self::position::{GridAutoFlow, GridTemplateAreas, Position, PositionOrAuto, ZIndex};
 pub use self::rect::NonNegativeLengthOrNumberRect;
 pub use self::resolution::Resolution;
 pub use self::svg::MozContextProperties;

components/style/values/computed/position.rs

@@ -9,6 +9,7 @@
 
 use crate::values::computed::{Integer, LengthPercentage, Percentage};
 use crate::values::generics::position::Position as GenericPosition;
+use crate::values::generics::position::PositionOrAuto as GenericPositionOrAuto;
 use crate::values::generics::position::ZIndex as GenericZIndex;
 pub use crate::values::specified::position::{GridAutoFlow, GridTemplateAreas};
 use crate::Zero;
@@ -18,6 +19,9 @@ use style_traits::{CssWriter, ToCss};
 /// The computed value of a CSS `<position>`
 pub type Position = GenericPosition<HorizontalPosition, VerticalPosition>;
 
+/// The computed value of an `auto | <position>`
+pub type PositionOrAuto = GenericPositionOrAuto<Position>;
+
 /// The computed value of a CSS horizontal position.
 pub type HorizontalPosition = LengthPercentage;
 

components/style/values/generics/position.rs

@@ -41,6 +41,40 @@ impl<H, V> Position<H, V> {
     }
 }
 
+/// A generic type for representing an `Auto | <position>`.
+/// This is used by <offset-anchor> for now.
+/// https://drafts.fxtf.org/motion-1/#offset-anchor-property
+#[derive(
+    Clone,
+    Copy,
+    Debug,
+    MallocSizeOf,
+    Parse,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToComputedValue,
+    ToCss,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[repr(C, u8)]
+pub enum GenericPositionOrAuto<Pos> {
+    /// The <position> value.
+    Position(Pos),
+    /// The keyword `auto`.
+    Auto,
+}
+
+pub use self::GenericPositionOrAuto as PositionOrAuto;
+
+impl<Pos> PositionOrAuto<Pos> {
+    /// Return `auto`.
+    #[inline]
+    pub fn auto() -> Self {
+        PositionOrAuto::Auto
+    }
+}
+
 /// A generic value for the `z-index` property.
 #[derive(
     Animate,

components/style/values/specified/mod.rs

@@ -69,7 +69,7 @@ pub use self::list::Quotes;
 pub use self::motion::{OffsetPath, OffsetRotate};
 pub use self::outline::OutlineStyle;
 pub use self::percentage::Percentage;
-pub use self::position::{GridAutoFlow, GridTemplateAreas, Position};
+pub use self::position::{GridAutoFlow, GridTemplateAreas, Position, PositionOrAuto};
 pub use self::position::{PositionComponent, ZIndex};
 pub use self::rect::NonNegativeLengthOrNumberRect;
 pub use self::resolution::Resolution;

components/style/values/specified/position.rs

@@ -13,6 +13,7 @@ use crate::str::HTML_SPACE_CHARACTERS;
 use crate::values::computed::LengthPercentage as ComputedLengthPercentage;
 use crate::values::computed::{Context, Percentage, ToComputedValue};
 use crate::values::generics::position::Position as GenericPosition;
+use crate::values::generics::position::PositionOrAuto as GenericPositionOrAuto;
 use crate::values::generics::position::ZIndex as GenericZIndex;
 use crate::values::specified::{AllowQuirks, Integer, LengthPercentage};
 use crate::Atom;
@@ -26,6 +27,9 @@ use style_traits::{CssWriter, ParseError, StyleParseErrorKind, ToCss};
 /// The specified value of a CSS `<position>`
 pub type Position = GenericPosition<HorizontalPosition, VerticalPosition>;
 
+/// The specified value of an `auto | <position>`.
+pub type PositionOrAuto = GenericPositionOrAuto<Position>;
+
 /// The specified value of a horizontal position.
 pub type HorizontalPosition = PositionComponent<HorizontalPositionKeyword>;