style: Record the property we are computing on computed::Context, if it's a non-inherited one.

components/style/gecko/media_queries.rs

@@ -706,6 +706,7 @@ impl Expression {
             // TODO: pass the correct value here.
             quirks_mode: quirks_mode,
             for_smil_animation: false,
+            for_non_inherited_property: None,
             rule_cache_conditions: RefCell::new(&mut conditions),
         };
 

components/style/properties/helpers.mako.rs

@@ -314,6 +314,13 @@
                 _ => panic!("entered the wrong cascade_property() implementation"),
             };
 
+            context.for_non_inherited_property =
+                % if property.style_struct.inherited:
+                    None;
+                % else:
+                    Some(LonghandId::${property.camel_case});
+                % endif
+
             % if not property.derived_from:
                 match value {
                     DeclaredValue::Value(specified_value) => {

components/style/properties/properties.mako.rs

@@ -3199,6 +3199,7 @@ where
         cached_system_font: None,
         in_media_query: false,
         for_smil_animation: false,
+        for_non_inherited_property: None,
         font_metrics_provider,
         quirks_mode,
         rule_cache_conditions: RefCell::new(rule_cache_conditions),

components/style/servo/media_queries.rs

@@ -260,6 +260,7 @@ impl Range<specified::Length> {
             cached_system_font: None,
             quirks_mode: quirks_mode,
             for_smil_animation: false,
+            for_non_inherited_property: None,
             rule_cache_conditions: RefCell::new(&mut conditions),
         };
 

components/style/stylesheets/viewport_rule.rs

@@ -718,6 +718,7 @@ impl MaybeNew for ViewportConstraints {
             in_media_query: false,
             quirks_mode: quirks_mode,
             for_smil_animation: false,
+            for_non_inherited_property: None,
             rule_cache_conditions: RefCell::new(&mut conditions),
         };
 

components/style/values/computed/mod.rs

@@ -11,7 +11,7 @@ use font_metrics::FontMetricsProvider;
 use media_queries::Device;
 #[cfg(feature = "gecko")]
 use properties;
-use properties::{ComputedValues, StyleBuilder};
+use properties::{ComputedValues, LonghandId, StyleBuilder};
 use rule_cache::RuleCacheConditions;
 #[cfg(feature = "servo")]
 use servo_url::ServoUrl;
@@ -119,6 +119,12 @@ pub struct Context<'a> {
     /// values, which SMIL allows.
     pub for_smil_animation: bool,
 
+    /// The property we are computing a value for, if it is a non-inherited
+    /// property.  None if we are computed a value for an inherited property
+    /// or not computing for a property at all (e.g. in a media query
+    /// evaluation).
+    pub for_non_inherited_property: Option<LonghandId>,
+
     /// The conditions to cache a rule node on the rule cache.
     ///
     /// FIXME(emilio): Drop the refcell.

ports/geckolib/glue.rs

@@ -3203,6 +3203,7 @@ fn create_context<'a>(
         in_media_query: false,
         quirks_mode: per_doc_data.stylist.quirks_mode(),
         for_smil_animation,
+        for_non_inherited_property: None,
         rule_cache_conditions: RefCell::new(rule_cache_conditions),
     }
 }