script: Reduce ScriptThread TLS usage (#38875)

We store a pointer to the ScriptThread singleton for a thread in
thread-local storage. While we don't have yet have profiles pointing to
this TLS access as a hot spot, we can remove a potential performance
footgun without a lot of effort by passing around small pieces of data
that we otherwise need to fetch from the ScriptThread.

Testing: Existing WPT is sufficient
Fixes: part of #37969

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/script/dom/document.rs

@@ -124,7 +124,7 @@ use crate::dom::cdatasection::CDATASection;
 use crate::dom::comment::Comment;
 use crate::dom::compositionevent::CompositionEvent;
 use crate::dom::cssstylesheet::CSSStyleSheet;
-use crate::dom::customelementregistry::CustomElementDefinition;
+use crate::dom::customelementregistry::{CustomElementDefinition, CustomElementReactionStack};
 use crate::dom::customevent::CustomEvent;
 use crate::dom::document_event_handler::DocumentEventHandler;
 use crate::dom::documentfragment::DocumentFragment;
@@ -565,6 +565,10 @@ pub(crate) struct Document {
     /// this `Document` will not perform any more rendering updates.
     #[no_trace]
     current_canvas_epoch: RefCell<Epoch>,
+
+    /// The global custom element reaction stack for this script thread.
+    #[conditional_malloc_size_of]
+    custom_element_reaction_stack: Rc<CustomElementReactionStack>,
 }
 
 #[allow(non_snake_case)]
@@ -3261,6 +3265,7 @@ impl Document {
         allow_declarative_shadow_roots: bool,
         inherited_insecure_requests_policy: Option<InsecureRequestsPolicy>,
         has_trustworthy_ancestor_origin: bool,
+        custom_element_reaction_stack: Rc<CustomElementReactionStack>,
     ) -> Document {
         let url = url.unwrap_or_else(|| ServoUrl::parse("about:blank").unwrap());
 
@@ -3434,6 +3439,7 @@ impl Document {
             resize_observer_started_observing_target: Cell::new(false),
             waiting_on_canvas_image_updates: Cell::new(false),
             current_canvas_epoch: RefCell::new(Epoch(0)),
+            custom_element_reaction_stack,
         }
     }
 
@@ -3536,6 +3542,7 @@ impl Document {
         allow_declarative_shadow_roots: bool,
         inherited_insecure_requests_policy: Option<InsecureRequestsPolicy>,
         has_trustworthy_ancestor_origin: bool,
+        custom_element_reaction_stack: Rc<CustomElementReactionStack>,
         can_gc: CanGc,
     ) -> DomRoot<Document> {
         Self::new_with_proto(
@@ -3557,6 +3564,7 @@ impl Document {
             allow_declarative_shadow_roots,
             inherited_insecure_requests_policy,
             has_trustworthy_ancestor_origin,
+            custom_element_reaction_stack,
             can_gc,
         )
     }
@@ -3581,6 +3589,7 @@ impl Document {
         allow_declarative_shadow_roots: bool,
         inherited_insecure_requests_policy: Option<InsecureRequestsPolicy>,
         has_trustworthy_ancestor_origin: bool,
+        custom_element_reaction_stack: Rc<CustomElementReactionStack>,
         can_gc: CanGc,
     ) -> DomRoot<Document> {
         let document = reflect_dom_object_with_proto(
@@ -3602,6 +3611,7 @@ impl Document {
                 allow_declarative_shadow_roots,
                 inherited_insecure_requests_policy,
                 has_trustworthy_ancestor_origin,
+                custom_element_reaction_stack,
             )),
             window,
             proto,
@@ -3736,6 +3746,7 @@ impl Document {
                     self.allow_declarative_shadow_roots(),
                     Some(self.insecure_requests_policy()),
                     self.has_trustworthy_ancestor_or_current_origin(),
+                    self.custom_element_reaction_stack.clone(),
                     can_gc,
                 );
                 new_doc
@@ -4413,6 +4424,10 @@ impl Document {
     pub(crate) fn highlighted_dom_node(&self) -> Option<DomRoot<Node>> {
         self.highlighted_dom_node.get()
     }
+
+    pub(crate) fn custom_element_reaction_stack(&self) -> Rc<CustomElementReactionStack> {
+        self.custom_element_reaction_stack.clone()
+    }
 }
 
 #[allow(non_snake_case)]
@@ -4444,6 +4459,7 @@ impl DocumentMethods<crate::DomTypeHolder> for Document {
             doc.allow_declarative_shadow_roots(),
             Some(doc.insecure_requests_policy()),
             doc.has_trustworthy_ancestor_or_current_origin(),
+            doc.custom_element_reaction_stack(),
             can_gc,
         ))
     }