style: Implement scroll-margin parser and serializer.

The reason why we use  RelaxedAtomBool is that
ScrollSnapUtils::GetSnapPointForDestination() is called both from the main and
the compositor threads, and the function will have a branch depending on the
pref value.

Differential Revision: https://phabricator.services.mozilla.com/D20101

components/style/properties/gecko.mako.rs

@@ -1661,14 +1661,19 @@ fn static_assert() {
     }
 </%self:impl_trait>
 
+<% skip_scroll_margin_longhands = " ".join(["scroll-margin-%s" % x.ident for x in SIDES]) %>
 <% skip_margin_longhands = " ".join(["margin-%s" % x.ident for x in SIDES]) %>
 <%self:impl_trait style_struct_name="Margin"
-                  skip_longhands="${skip_margin_longhands}">
+                  skip_longhands="${skip_margin_longhands}
+                                  ${skip_scroll_margin_longhands}">
 
     % for side in SIDES:
     <% impl_split_style_coord("margin_%s" % side.ident,
                               "mMargin",
                               side.index) %>
+    <% impl_split_style_coord("scroll_margin_%s" % side.ident,
+                              "mScrollMargin",
+                              side.index) %>
     % endfor
 </%self:impl_trait>
 

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

@@ -27,3 +27,18 @@
         servo_restyle_damage="reflow"
     )}
 % endfor
+
+% for side in ALL_SIDES:
+    ${helpers.predefined_type(
+        "scroll-margin-%s" % side[0],
+        "Length",
+        "computed::Length::zero()",
+        products="gecko",
+        gecko_pref="layout.css.scroll-snap-v1.enabled",
+        logical=side[1],
+        logical_group="scroll-margin",
+        spec="https://drafts.csswg.org/css-scroll-snap-1/#propdef-scroll-margin-%s" % side[0],
+        flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE",
+        animation_value_type="ComputedValue",
+    )}
+% endfor

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

@@ -28,3 +28,32 @@ ${helpers.two_properties_shorthand(
     "specified::LengthPercentageOrAuto::parse",
     spec="https://drafts.csswg.org/css-logical/#propdef-margin-inline"
 )}
+
+${helpers.four_sides_shorthand(
+    "scroll-margin",
+    "scroll-margin-%s",
+    "specified::Length::parse",
+    spec="https://drafts.csswg.org/css-scroll-snap-1/#propdef-scroll-margin",
+    products="gecko",
+    gecko_pref="layout.css.scroll-snap-v1.enabled",
+)}
+
+${helpers.two_properties_shorthand(
+    "scroll-margin-block",
+    "scroll-margin-block-start",
+    "scroll-margin-block-end",
+    "specified::Length::parse",
+    spec="https://drafts.csswg.org/css-scroll-snap-1/#propdef-scroll-margin-block",
+    products="gecko",
+    gecko_pref="layout.css.scroll-snap-v1.enabled",
+)}
+
+${helpers.two_properties_shorthand(
+    "scroll-margin-inline",
+    "scroll-margin-inline-start",
+    "scroll-margin-inline-end",
+    "specified::Length::parse",
+    spec="https://drafts.csswg.org/css-scroll-snap-1/#propdef-scroll-margin-inline",
+    products="gecko",
+    gecko_pref="layout.css.scroll-snap-v1.enabled",
+)}