style: Invalidate parts in nested shadow trees correctly.

Differential Revision: https://phabricator.services.mozilla.com/D54010

1 files changed, 14 insertions, 4 deletions

diff --git a/components/style/invalidation/element/element_wrapper.rs b/components/style/invalidation/element/element_wrapper.rs
index e17e42a31c0..bc74527bf16 100644
--- a/components/style/invalidation/element/element_wrapper.rs
+++ b/components/style/invalidation/element/element_wrapper.rs
@@ -62,6 +62,12 @@ pub trait ElementSnapshot: Sized {
     /// called if `has_attrs()` returns true.
     fn is_part(&self, name: &Atom) -> bool;
 
+    /// See Element::exported_part.
+    fn exported_part(&self, name: &Atom) -> Option<Atom>;
+
+    /// See Element::imported_part.
+    fn imported_part(&self, name: &Atom) -> Option<Atom>;
+
     /// A callback that should be called for each class of the snapshot. Should
     /// only be called if `has_attrs()` returns true.
     fn each_class<F>(&self, _: F)
@@ -366,13 +372,17 @@ where
     }
 
     fn exported_part(&self, name: &Atom) -> Option<Atom> {
-        // FIXME(emilio): Implement for proper invalidation.
-        self.element.exported_part(name)
+        match self.snapshot() {
+            Some(snapshot) if snapshot.has_attrs() => snapshot.exported_part(name),
+            _ => self.element.exported_part(name),
+        }
     }
 
     fn imported_part(&self, name: &Atom) -> Option<Atom> {
-        // FIXME(emilio): Implement for proper invalidation.
-        self.element.imported_part(name)
+        match self.snapshot() {
+            Some(snapshot) if snapshot.has_attrs() => snapshot.imported_part(name),
+            _ => self.element.imported_part(name),
+        }
     }
 
     fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool {