Adds scroll-snap-type shorthand property, tests

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

@@ -297,3 +297,37 @@ macro_rules! try_parse_one {
         }
     }
 </%helpers:shorthand>
+
+<%helpers:shorthand name="scroll-snap-type" products="gecko"
+                    sub_properties="scroll-snap-type-x scroll-snap-type-y">
+    use properties::longhands::scroll_snap_type_x;
+
+    pub fn parse_value(context: &ParserContext, input: &mut Parser) -> Result<Longhands, ()> {
+        let result = try!(scroll_snap_type_x::parse(context, input));
+        Ok(Longhands {
+            scroll_snap_type_x: Some(result),
+            scroll_snap_type_y: Some(result),
+        })
+    }
+
+    impl<'a> LonghandsToSerialize<'a>  {
+        // Serializes into the single keyword value if both scroll-snap-type and scroll-snap-type-y are same.
+        // Otherwise into an empty string. This is done to match Gecko's behaviour.
+        fn to_css_declared<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
+            let x_and_y_equal = match (self.scroll_snap_type_x, self.scroll_snap_type_y) {
+                (&DeclaredValue::Value(ref x_value), &DeclaredValue::Value(ref y_value)) => {
+                    *x_value == *y_value
+                },
+                (&DeclaredValue::Initial, &DeclaredValue::Initial) => true,
+                (&DeclaredValue::Inherit, &DeclaredValue::Inherit) => true,
+                (x, y) => { *x == *y },
+            };
+
+            if x_and_y_equal {
+                self.scroll_snap_type_x.to_css(dest)
+            } else {
+                Ok(())
+            }
+        }
+    }
+</%helpers:shorthand>

tests/unit/style/properties/serialization.rs

@@ -1006,4 +1006,53 @@ mod shorthand_serialization {
             );
         }
     }
+
+    mod scroll_snap_type {
+        pub use super::*;
+        use style::properties::longhands::scroll_snap_type_x::computed_value::T as ScrollSnapTypeXValue;
+
+        #[test]
+        fn should_serialize_to_empty_string_if_sub_types_not_equal() {
+            let declarations = vec![
+                (PropertyDeclaration::ScrollSnapTypeX(DeclaredValue::Value(ScrollSnapTypeXValue::mandatory)),
+                Importance::Normal),
+                (PropertyDeclaration::ScrollSnapTypeY(DeclaredValue::Value(ScrollSnapTypeXValue::none)),
+                Importance::Normal)
+            ];
+
+            let block = PropertyDeclarationBlock {
+                declarations: declarations,
+                important_count: 0
+            };
+
+            let mut s = String::new();
+
+            let x = block.single_value_to_css("scroll-snap-type", &mut s);
+
+            assert_eq!(x.is_ok(), true);
+            assert_eq!(s, "scroll-snap-type: ;");
+        }
+
+        #[test]
+        fn should_serialize_to_single_value_if_sub_types_are_equal() {
+            let declarations = vec![
+                (PropertyDeclaration::ScrollSnapTypeX(DeclaredValue::Value(ScrollSnapTypeXValue::mandatory)),
+                Importance::Normal),
+                (PropertyDeclaration::ScrollSnapTypeY(DeclaredValue::Value(ScrollSnapTypeXValue::mandatory)),
+                Importance::Normal)
+            ];
+
+            let block = PropertyDeclarationBlock {
+                declarations: declarations,
+                important_count: 0
+            };
+
+            let mut s = String::new();
+
+            let x = block.single_value_to_css("scroll-snap-type", &mut s);
+
+            assert_eq!(x.is_ok(), true);
+            assert_eq!(s, "scroll-snap-type: mandatory;");
+        }
+    }
 }