style: Don't share style with an element that has a different proto binding than us.

See the lengthy comment about why... XBL sucks.

MozReview-Commit-ID: 8UgeJAVkj6a

components/style/dom.rs

@@ -739,6 +739,9 @@ pub trait TElement
     /// The shadow root which roots the subtree this element is contained in.
     fn containing_shadow(&self) -> Option<<Self::ConcreteNode as TNode>::ConcreteShadowRoot>;
 
+    /// XBL hack for style sharing. :(
+    fn has_same_xbl_proto_binding_as(&self, _other: Self) -> bool { true }
+
     /// Return the element which we can use to look up rules in the selector
     /// maps.
     ///

components/style/gecko/wrapper.rs

@@ -1092,8 +1092,16 @@ impl<'le> TElement for GeckoElement<'le> {
         unsafe { slots._base.mContainingShadow.mRawPtr.as_ref().map(GeckoShadowRoot) }
     }
 
-    /// Execute `f` for each anonymous content child element (apart from
-    /// ::before and ::after) whose originating element is `self`.
+    fn has_same_xbl_proto_binding_as(&self, other: Self) -> bool {
+        match (self.xbl_binding(), other.xbl_binding()) {
+            (None, None) => true,
+            (Some(a), Some(b)) => {
+                a.0.mPrototypeBinding == b.0.mPrototypeBinding
+            }
+            _ => false,
+        }
+    }
+
     fn each_anonymous_content_child<F>(&self, mut f: F)
     where
         F: FnMut(Self),

components/style/sharing/mod.rs

@@ -681,15 +681,29 @@ impl<E: TElement> StyleSharingCache<E> {
 
         // If two elements belong to different shadow trees, different rules may
         // apply to them, from the respective trees.
-        //
-        // Note that we don't need the same for XBL case, since two elements
-        // with different XBL bindings need to necessarily have different style
-        // (and thus children of them would never pass the parent check).
         if target.element.containing_shadow() != candidate.element.containing_shadow() {
             trace!("Miss: Different containing shadow roots");
             return None;
         }
 
+        // Note that in theory we shouldn't need this XBL check. However, XBL is
+        // absolutely broken in all sorts of ways.
+        //
+        // A style change that changes which XBL binding applies to an element
+        // arrives there, with the element still having the old prototype
+        // binding attached. And thus we try to match revalidation selectors
+        // with the old XBL binding, because we can't look at the new ones of
+        // course. And that causes us to revalidate with the wrong selectors and
+        // hit assertions.
+        //
+        // Other than this, we don't need anything else like the containing XBL
+        // binding parent or what not, since two elements with different XBL
+        // bindings will necessarily end up with different style.
+        if !target.element.has_same_xbl_proto_binding_as(candidate.element) {
+            trace!("Miss: Different proto bindings");
+            return None;
+        }
+
         // If the elements are not assigned to the same slot they could match
         // different ::slotted() rules in the slot scope.
         //