Auto merge of #13312 - 6112:flush-style-cache, r=emilio

Flush style sharing cache on parent mismatch.

Fixes #12913. No new tests were added, but we should at least make sure existing tests still pass.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #12913

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- [ ] These changes do not require tests because _____

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components/style/matching.rs

@@ -708,6 +708,7 @@ pub trait ElementMatchMethods : TElement {
             _ => return StyleSharingResult::CannotShare,
         };
 
+        let mut should_clear_cache = false;
         for (i, &mut (ref mut candidate, ())) in style_sharing_candidate_cache.iter_mut().enumerate() {
             let sharing_result = self.share_style_with_candidate_if_possible(parent,
                                                                              shared_context,
@@ -750,6 +751,11 @@ pub trait ElementMatchMethods : TElement {
                     // Cache miss, let's see what kind of failure to decide
                     // whether we keep trying or not.
                     match miss {
+                        // Cache miss because of parent, clear the candidate cache.
+                        CacheMiss::Parent => {
+                            should_clear_cache = true;
+                            break;
+                        },
                         // Too expensive failure, give up, we don't want another
                         // one of these.
                         CacheMiss::CommonStyleAffectingAttributes |
@@ -761,6 +767,9 @@ pub trait ElementMatchMethods : TElement {
                 }
             }
         }
+        if should_clear_cache {
+            style_sharing_candidate_cache.clear();
+        }
 
         StyleSharingResult::CannotShare
     }