Avoid leaving stale ANCESTOR_WAS_RECONSTRUCTED bits in the tree.

MozReview-Commit-ID: 76q5XxK2o2a

2 files changed, 15 insertions, 7 deletions

diff --git a/components/style/data.rs b/components/style/data.rs
index 096f09f00de..35f75b10b3d 100644
--- a/components/style/data.rs
+++ b/components/style/data.rs
@@ -106,10 +106,14 @@ impl RestyleData {
     }
 
     /// Sets the flag that tells us whether we've reconstructed an ancestor.
-    pub fn set_reconstructed_ancestor(&mut self) {
-        // If it weren't for animation-only traversals, we could assert
-        // `!self.reconstructed_ancestor()` here.
-        self.flags.insert(ANCESTOR_WAS_RECONSTRUCTED);
+    pub fn set_reconstructed_ancestor(&mut self, reconstructed: bool) {
+        if reconstructed {
+            // If it weren't for animation-only traversals, we could assert
+            // `!self.reconstructed_ancestor()` here.
+            self.flags.insert(ANCESTOR_WAS_RECONSTRUCTED);
+        } else {
+            self.flags.remove(ANCESTOR_WAS_RECONSTRUCTED);
+        }
     }
 
     /// Mark this element as restyled, which is useful to know whether we need
diff --git a/components/style/traversal.rs b/components/style/traversal.rs
index 546e254adf4..c6bf7e8ee39 100644
--- a/components/style/traversal.rs
+++ b/components/style/traversal.rs
@@ -170,6 +170,12 @@ pub trait DomTraversal<E: TElement> : Sync {
             // Invalidate our style, and the one of our siblings and descendants
             // as needed.
             data.invalidate_style_if_needed(root, shared_context);
+
+            // Make sure we don't have any stale RECONSTRUCTED_ANCESTOR bits from
+            // the last traversal (at a potentially-higher root). From the
+            // perspective of this traversal, the root cannot have reconstructed
+            // ancestors.
+            data.restyle.set_reconstructed_ancestor(false);
         };
 
         let parent = root.traversal_parent();
@@ -793,9 +799,7 @@ where
         if let Some(ref mut child_data) = child_data {
             // Propagate the parent restyle hint, that may make us restyle the whole
             // subtree.
-            if reconstructed_ancestor {
-                child_data.restyle.set_reconstructed_ancestor();
-            }
+            child_data.restyle.set_reconstructed_ancestor(reconstructed_ancestor);
 
             child_data.restyle.hint.insert(propagated_hint);