style: Update the syntax of offset-position

Now it supports "normal" keyword. Also, offset-position doesn't create
stacking context and it doesn't have offset transform, so we can
simplify CompareMotionValues() a little bit.

Note: We don't have to add test in [1] because Blink added one to WPT
upstream repo already.

[1] css/motion/parsing/offset-position-parsing-valid.html

Note: the usage of offset-position is in other bugs.

Differential Revision: https://phabricator.services.mozilla.com/D179623

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

@@ -268,8 +268,8 @@ ${helpers.predefined_type(
 // Motion Path Module Level 1
 ${helpers.predefined_type(
     "offset-position",
-    "PositionOrAuto",
-    "computed::PositionOrAuto::auto()",
+    "OffsetPosition",
+    "computed::OffsetPosition::auto()",
     engines="gecko",
     animation_value_type="ComputedValue",
     gecko_pref="layout.css.motion-path-offset-position.enabled",

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

@@ -152,9 +152,8 @@ ${helpers.two_properties_shorthand(
                     gecko_pref="layout.css.motion-path.enabled",
                     spec="https://drafts.fxtf.org/motion-1/#offset-shorthand">
     use crate::parser::Parse;
-    use crate::values::specified::motion::{OffsetPath, OffsetRotate};
-    use crate::values::specified::position::PositionOrAuto;
-    use crate::values::specified::LengthPercentage;
+    use crate::values::specified::motion::{OffsetPath, OffsetPosition, OffsetRotate};
+    use crate::values::specified::{LengthPercentage, PositionOrAuto};
     use crate::Zero;
 
     pub fn parse_value<'i, 't>(
@@ -163,7 +162,7 @@ ${helpers.two_properties_shorthand(
     ) -> Result<Longhands, ParseError<'i>> {
         let offset_position =
             if static_prefs::pref!("layout.css.motion-path-offset-position.enabled") {
-                input.try_parse(|i| PositionOrAuto::parse(context, i)).ok()
+                input.try_parse(|i| OffsetPosition::parse(context, i)).ok()
             } else {
                 None
             };
@@ -171,6 +170,8 @@ ${helpers.two_properties_shorthand(
         let offset_path = input.try_parse(|i| OffsetPath::parse(context, i)).ok();
 
         // Must have one of [offset-position, offset-path].
+        // FIXME: The syntax is out-of-date after the update of the spec.
+        // https://github.com/w3c/fxtf-drafts/issues/515
         if offset_position.is_none() && offset_path.is_none() {
             return Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError));
         }
@@ -202,7 +203,7 @@ ${helpers.two_properties_shorthand(
         }).ok();
 
         Ok(expanded! {
-            offset_position: offset_position.unwrap_or(PositionOrAuto::auto()),
+            offset_position: offset_position.unwrap_or(OffsetPosition::auto()),
             offset_path: offset_path.unwrap_or(OffsetPath::none()),
             offset_distance: offset_distance.unwrap_or(LengthPercentage::zero()),
             offset_rotate: offset_rotate.unwrap_or(OffsetRotate::auto()),
@@ -217,7 +218,7 @@ ${helpers.two_properties_shorthand(
                 // offset-path group means "offset-path offset-distance offset-rotate".
                 let must_serialize_path = *self.offset_path != OffsetPath::None
                     || (!self.offset_distance.is_zero() || !self.offset_rotate.is_auto());
-                let position_is_default = *offset_position == PositionOrAuto::auto();
+                let position_is_default = matches!(offset_position, OffsetPosition::Auto);
                 if !position_is_default || !must_serialize_path {
                     offset_position.to_css(dest)?;
                 }

components/style/values/computed/mod.rs

@@ -79,7 +79,7 @@ pub use self::length::{NonNegativeLengthPercentage, NonNegativeLengthPercentageO
 #[cfg(feature = "gecko")]
 pub use self::list::ListStyleType;
 pub use self::list::Quotes;
-pub use self::motion::{OffsetPath, OffsetRotate};
+pub use self::motion::{OffsetPath, OffsetPosition, OffsetRotate};
 pub use self::outline::OutlineStyle;
 pub use self::page::{PageName, PageOrientation, PageSize, PageSizeOrientation, PaperSize};
 pub use self::percentage::{NonNegativePercentage, Percentage};

components/style/values/computed/motion.rs

@@ -4,13 +4,16 @@
 
 //! Computed types for CSS values that are related to motion path.
 
-use crate::values::computed::Angle;
-use crate::values::generics::motion::GenericOffsetPath;
+use crate::values::computed::{Angle, LengthPercentage};
+use crate::values::generics::motion::{GenericOffsetPath, GenericOffsetPosition};
 use crate::Zero;
 
 /// The computed value of `offset-path`.
 pub type OffsetPath = GenericOffsetPath<Angle>;
 
+/// The computed value of `offset-position`.
+pub type OffsetPosition = GenericOffsetPosition<LengthPercentage, LengthPercentage>;
+
 #[inline]
 fn is_auto_zero_angle(auto: &bool, angle: &Angle) -> bool {
     *auto && angle.is_zero()

components/style/values/generics/motion.rs

@@ -4,6 +4,7 @@
 
 //! Generic types for CSS Motion Path.
 
+use crate::values::generics::position::GenericPosition;
 use crate::values::specified::SVGPathData;
 
 /// The <size> in ray() function.
@@ -123,3 +124,51 @@ impl<Angle> OffsetPath<Angle> {
         OffsetPath::None
     }
 }
+
+/// The offset-position property, which specifies the offset starting position that is used by the
+/// <offset-path> functions if they don’t specify their own starting position.
+///
+/// https://drafts.fxtf.org/motion-1/#offset-position-property
+#[derive(
+    Animate,
+    Clone,
+    ComputeSquaredDistance,
+    Copy,
+    Debug,
+    Deserialize,
+    MallocSizeOf,
+    Parse,
+    PartialEq,
+    Serialize,
+    SpecifiedValueInfo,
+    ToAnimatedValue,
+    ToAnimatedZero,
+    ToComputedValue,
+    ToCss,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[repr(C, u8)]
+pub enum GenericOffsetPosition<H, V> {
+    /// The element does not have an offset starting position.
+    Normal,
+    /// The offset starting position is the top-left corner of the box.
+    Auto,
+    /// The offset starting position is the result of using the <position> to position a 0x0 object
+    /// area within the box’s containing block.
+    Position(
+        #[css(field_bound)]
+        #[parse(field_bound)]
+        GenericPosition<H, V>,
+    ),
+}
+
+pub use self::GenericOffsetPosition as OffsetPosition;
+
+impl<H, V> OffsetPosition<H, V> {
+    /// Returns the initial value, auto.
+    #[inline]
+    pub fn auto() -> Self {
+        Self::Auto
+    }
+}

components/style/values/specified/mod.rs

@@ -73,7 +73,7 @@ pub use self::length::{
 #[cfg(feature = "gecko")]
 pub use self::list::ListStyleType;
 pub use self::list::Quotes;
-pub use self::motion::{OffsetPath, OffsetRotate};
+pub use self::motion::{OffsetPath, OffsetPosition, OffsetRotate};
 pub use self::outline::OutlineStyle;
 pub use self::page::{PageName, PageOrientation, PageSize, PageSizeOrientation, PaperSize};
 pub use self::percentage::{NonNegativePercentage, Percentage};

components/style/values/specified/motion.rs

@@ -7,7 +7,10 @@
 use crate::parser::{Parse, ParserContext};
 use crate::values::computed::motion::OffsetRotate as ComputedOffsetRotate;
 use crate::values::computed::{Context, ToComputedValue};
-use crate::values::generics::motion::{GenericOffsetPath, RayFunction, RaySize};
+use crate::values::generics::motion::{
+    GenericOffsetPath, GenericOffsetPosition, RayFunction, RaySize,
+};
+use crate::values::specified::position::{HorizontalPosition, VerticalPosition};
 use crate::values::specified::{Angle, SVGPathData};
 use crate::Zero;
 use cssparser::Parser;
@@ -16,6 +19,9 @@ use style_traits::{ParseError, StyleParseErrorKind};
 /// The specified value of `offset-path`.
 pub type OffsetPath = GenericOffsetPath<Angle>;
 
+/// The specified value of `offset-position`.
+pub type OffsetPosition = GenericOffsetPosition<HorizontalPosition, VerticalPosition>;
+
 #[cfg(feature = "gecko")]
 fn is_ray_enabled() -> bool {
     static_prefs::pref!("layout.css.motion-path-ray.enabled")