Bug 1366427: Make backdrop not inherit from the parent element. r=heycam

MozReview-Commit-ID: EnnmvDsgbdx
Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

components/style/gecko/pseudo_element.rs

@@ -49,6 +49,15 @@ impl PseudoElement {
         PseudoElementCascadeType::Lazy
     }
 
+    /// Whether the pseudo-element should inherit from the default computed
+    /// values instead of from the parent element.
+    ///
+    /// This is not the common thing, but there are some pseudos (namely:
+    /// ::backdrop), that shouldn't inherit from the parent element.
+    pub fn inherits_from_default_values(&self) -> bool {
+        !matches!(*self, PseudoElement::Backdrop)
+    }
+
     /// Gets the canonical index of this eagerly-cascaded pseudo-element.
     #[inline]
     pub fn eager_index(&self) -> usize {

components/style/stylist.rs

@@ -630,7 +630,7 @@ impl Stylist {
                                                   guards: &StylesheetGuards,
                                                   element: &E,
                                                   pseudo: &PseudoElement,
-                                                  parent: &Arc<ComputedValues>,
+                                                  parent_style: &ComputedValues,
                                                   font_metrics: &FontMetricsProvider)
                                                   -> Option<ComputedStyle>
         where E: TElement,
@@ -649,8 +649,8 @@ impl Stylist {
             properties::cascade(&self.device,
                                 &rule_node,
                                 guards,
-                                Some(&**parent),
+                                Some(parent_style),
-                                Some(&**parent),
+                                Some(parent_style),
                                 None,
                                 &RustLogReporter,
                                 font_metrics,

ports/geckolib/glue.rs

@@ -1205,7 +1205,11 @@ fn get_pseudo_style(guard: &SharedRwLockReadGuard,
         PseudoElementCascadeType::Precomputed => unreachable!("No anonymous boxes"),
         PseudoElementCascadeType::Lazy => {
             let d = doc_data.borrow_mut();
-            let base = styles.primary.values();
+            let base = if pseudo.inherits_from_default_values() {
+                d.default_computed_values()
+            } else {
+                styles.primary.values()
+            };
             let guards = StylesheetGuards::same(guard);
             let metrics = get_metrics_provider_for_product();
             d.stylist.lazily_compute_pseudo_element_style(&guards,