Keep a list of slot descendants on each shadow root (#35802)

This makes it much faster to traverse over the slot descendants of
a shadow root, which is a fairly costly part of "assign slottables to a tree".

This reduces the time it takes for the results to load on wpt.fyi from
over 3 minutes to about 5 seconds.

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

1 files changed, 5 insertions, 1 deletions

diff --git a/components/script/dom/node.rs b/components/script/dom/node.rs
index d4e404dc030..4a4c9f53462 100644
--- a/components/script/dom/node.rs
+++ b/components/script/dom/node.rs
@@ -1362,7 +1362,11 @@ impl Node {
         // NOTE: This method traverses all descendants of the node and is potentially very
         // expensive. If the node is not a shadow root then assigning slottables to it won't
         // have any effect, so we take a fast path out.
-        if !self.is::<ShadowRoot>() {
+        let Some(shadow_root) = self.downcast::<ShadowRoot>() else {
+            return;
+        };
+
+        if !shadow_root.has_slot_descendants() {
             return;
         }