servo: Merge #19432 - style: Bail out from invalidation if we're the root and got tons of descendant invalidations (from emilio:invalidation-bail); r=heycam

Bug: 1420741
Reviewed-by: heycam
MozReview-Commit-ID: 4Kja20Ep9qD
Source-Repo: https://github.com/servo/servo
Source-Revision: e4d5e9699b9e784e5eba0aec316a35d048786052

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 705ee3ef74c8f09702fa0e09bd9924f58bfc8ac7

servo/components/style/invalidation/element/collector.rs

@@ -229,6 +229,17 @@ where
             collector.invalidates_self
         };
 
+        // If we generated a ton of descendant invalidations, it's probably not
+        // worth to go ahead and try to process them.
+        //
+        // Just restyle the descendants directly.
+        //
+        // This number is completely made-up, but the page that made us add this
+        // code generated 1960+ invalidations (bug 1420741).
+        if descendant_invalidations.len() > 150 {
+            self.data.hint.insert(RestyleHint::RESTYLE_DESCENDANTS);
+        }
+
         if invalidated_self {
             self.data.hint.insert(RestyleHint::RESTYLE_SELF);
         }