style: when iterating over a selector to find a bucket, choose the rightmost

This restores the pre-regression behavior by choosing the later class in
cases where folks use stuff like `.foo.bar`.

This matches other browsers too.

Differential Revision: https://phabricator.services.mozilla.com/D177398

1 files changed, 8 insertions, 1 deletions

diff --git a/components/style/selector_map.rs b/components/style/selector_map.rs
index 3b7515292bf..732168cbfd8 100644
--- a/components/style/selector_map.rs
+++ b/components/style/selector_map.rs
@@ -685,6 +685,11 @@ impl<'a> Bucket<'a> {
     }
 
     #[inline]
+    fn more_or_equally_specific_than(&self, other: &Self) -> bool {
+        self.specificity() >= other.specificity()
+    }
+
+    #[inline]
     fn more_specific_than(&self, other: &Self) -> bool {
         self.specificity() > other.specificity()
     }
@@ -787,7 +792,9 @@ fn find_bucket<'a>(
     loop {
         for ss in &mut iter {
             let new_bucket = specific_bucket_for(ss, disjoint_buckets, bucket_attributes);
-            if new_bucket.more_specific_than(&current_bucket) {
+            // NOTE: When presented with the choice of multiple specific selectors, use the
+            // rightmost, on the assumption that that's less common, see bug 1829540.
+            if new_bucket.more_or_equally_specific_than(&current_bucket) {
                 current_bucket = new_bucket;
             }
         }