From 033da09800e4f7acce36f0b5a346d84dd5455c57 Mon Sep 17 00:00:00 2001
From: Narfinger <Narfinger@users.noreply.github.com>
Date: Fri, 12 Sep 2025 20:43:59 +0200
Subject: [PATCH] Move signals_slots to ScriptMutationObservers (#39275)

Both places where we access signals_slots already have a reference to
ScriptMutationObserver. This saves us another access to
with_script_thread.


Testing: This does not change functionality.
Fixes: Part of addressing: https://github.com/servo/servo/issues/37969

Signed-off-by: Narfinger <Narfinger@users.noreply.github.com>
---
 components/script/dom/html/htmlslotelement.rs |  4 +--
 .../script/script_mutation_observers.rs       | 21 ++++++++++++--
 components/script/script_thread.rs            | 28 -------------------
 3 files changed, 21 insertions(+), 32 deletions(-)

diff --git a/components/script/dom/html/htmlslotelement.rs b/components/script/dom/html/htmlslotelement.rs
index e77cf89c30b..35153a9e1f9 100644
--- a/components/script/dom/html/htmlslotelement.rs
+++ b/components/script/dom/html/htmlslotelement.rs
@@ -352,11 +352,11 @@ impl HTMLSlotElement {
         }
         self.is_in_agents_signal_slots.set(true);
 
+        let mutation_observers = ScriptThread::mutation_observers();
         // Step 1. Append slot to slot’s relevant agent’s signal slots.
-        ScriptThread::add_signal_slot(self);
+        mutation_observers.add_signal_slot(self);
 
         // Step 2. Queue a mutation observer microtask.
-        let mutation_observers = ScriptThread::mutation_observers();
         mutation_observers.queue_mutation_observer_microtask(ScriptThread::microtask_queue());
     }
 
diff --git a/components/script/script_mutation_observers.rs b/components/script/script_mutation_observers.rs
index 670e45b5a52..5134712d05e 100644
--- a/components/script/script_mutation_observers.rs
+++ b/components/script/script_mutation_observers.rs
@@ -10,7 +10,6 @@ use script_bindings::inheritance::Castable;
 use script_bindings::root::{Dom, DomRoot};
 use script_bindings::script_runtime::CanGc;
 
-use crate::ScriptThread;
 use crate::dom::bindings::cell::DomRefCell;
 use crate::dom::types::{EventTarget, HTMLSlotElement, MutationObserver, MutationRecord};
 use crate::microtask::{Microtask, MicrotaskQueue};
@@ -26,6 +25,9 @@ pub(crate) struct ScriptMutationObservers {
 
     /// The unit of related similar-origin browsing contexts' list of MutationObserver objects
     mutation_observers: DomRefCell<Vec<Dom<MutationObserver>>>,
+
+    /// <https://dom.spec.whatwg.org/#signal-slot-list>
+    signal_slots: DomRefCell<Vec<Dom<HTMLSlotElement>>>,
 }
 
 impl ScriptMutationObservers {
@@ -46,7 +48,7 @@ impl ScriptMutationObservers {
 
         // Step 4. Let signalSet be a clone of the surrounding agent’s signal slots.
         // Step 5. Empty the surrounding agent’s signal slots.
-        let signal_set: Vec<DomRoot<HTMLSlotElement>> = ScriptThread::take_signal_slots();
+        let signal_set: Vec<DomRoot<HTMLSlotElement>> = self.take_signal_slots();
 
         // Step 6. For each mo of notifySet:
         for mo in notify_list.iter() {
@@ -93,4 +95,19 @@ impl ScriptMutationObservers {
             microtask_queue.enqueue(Microtask::NotifyMutationObservers, script_thread.get_cx());
         });
     }
+
+    pub(crate) fn add_signal_slot(&self, observer: &HTMLSlotElement) {
+        self.signal_slots.borrow_mut().push(Dom::from_ref(observer));
+    }
+
+    pub(crate) fn take_signal_slots(&self) -> Vec<DomRoot<HTMLSlotElement>> {
+        self.signal_slots
+            .take()
+            .into_iter()
+            .inspect(|slot| {
+                slot.remove_from_signal_slots();
+            })
+            .map(|slot| slot.as_rooted())
+            .collect()
+    }
 }
diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index 92a4070b4bf..280a47801a6 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -126,7 +126,6 @@ use crate::dom::document::{
 use crate::dom::element::Element;
 use crate::dom::globalscope::GlobalScope;
 use crate::dom::html::htmliframeelement::HTMLIFrameElement;
-use crate::dom::html::htmlslotelement::HTMLSlotElement;
 use crate::dom::node::NodeTraits;
 use crate::dom::servoparser::{ParserContext, ServoParser};
 use crate::dom::types::DebuggerGlobalScope;
@@ -281,9 +280,6 @@ pub struct ScriptThread {
 
     mutation_observers: Rc<ScriptMutationObservers>,
 
-    /// <https://dom.spec.whatwg.org/#signal-slot-list>
-    signal_slots: DomRefCell<Vec<Dom<HTMLSlotElement>>>,
-
     /// A handle to the WebGL thread
     #[no_trace]
     webgl_chan: Option<WebGLPipeline>,
@@ -507,29 +503,6 @@ impl ScriptThread {
         with_script_thread(|script_thread| script_thread.microtask_queue.clone())
     }
 
-    pub(crate) fn add_signal_slot(observer: &HTMLSlotElement) {
-        with_script_thread(|script_thread| {
-            script_thread
-                .signal_slots
-                .borrow_mut()
-                .push(Dom::from_ref(observer));
-        })
-    }
-
-    pub(crate) fn take_signal_slots() -> Vec<DomRoot<HTMLSlotElement>> {
-        with_script_thread(|script_thread| {
-            script_thread
-                .signal_slots
-                .take()
-                .into_iter()
-                .inspect(|slot| {
-                    slot.remove_from_signal_slots();
-                })
-                .map(|slot| slot.as_rooted())
-                .collect()
-        })
-    }
-
     pub(crate) fn mark_document_with_no_blocked_loads(doc: &Document) {
         with_script_thread(|script_thread| {
             script_thread
@@ -1001,7 +974,6 @@ impl ScriptThread {
             js_runtime,
             closed_pipelines: DomRefCell::new(FxHashSet::default()),
             mutation_observers: Default::default(),
-            signal_slots: Default::default(),
             system_font_service,
             webgl_chan: state.webgl_chan,
             #[cfg(feature = "webxr")]