Fire slot change events when the slot content changes (#35137)

* Add the onslotchange attribute to ShadowRoot

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

* Add spec comments to MutationObserver::queue_mutation_observer_microtask

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

* Add DomRefCell::take

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

* Add spec comments to notify_mutation_observers

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

* Fire slotchange events when a slot changes

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

* ./mach fmt

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

* Fix check for when to dispatch slot events

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

* Potentially fire slot change events in Node::remove

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

* Update WPT expectations

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

* ./mach fmt

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

* Bump stylo

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

* Move "signal a slot change" into ScriptThread impl

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

---------

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

7 files changed, 135 insertions, 12 deletions

diff --git a/components/script/dom/bindings/cell.rs b/components/script/dom/bindings/cell.rs
index df0ca84b060..f0f630672c1 100644
--- a/components/script/dom/bindings/cell.rs
+++ b/components/script/dom/bindings/cell.rs
@@ -151,3 +151,14 @@ impl<T> DomRefCell<T> {
         self.value.try_borrow_mut()
     }
 }
+
+impl<T: Default> DomRefCell<T> {
+    /// Takes the wrapped value, leaving `Default::default()` in its place.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the value is currently borrowed.
+    pub(crate) fn take(&self) -> T {
+        self.value.take()
+    }
+}
diff --git a/components/script/dom/htmlslotelement.rs b/components/script/dom/htmlslotelement.rs
index b5fcdb91c6d..f60d5f3b5b1 100644
--- a/components/script/dom/htmlslotelement.rs
+++ b/components/script/dom/htmlslotelement.rs
@@ -32,6 +32,7 @@ use crate::dom::node::{Node, ShadowIncluding};
 use crate::dom::text::Text;
 use crate::dom::virtualmethods::VirtualMethods;
 use crate::script_runtime::CanGc;
+use crate::ScriptThread;
 
 /// <https://html.spec.whatwg.org/multipage/#the-slot-element>
 #[dom_struct]
@@ -185,6 +186,10 @@ impl HTMLSlotElement {
         )
     }
 
+    pub(crate) fn has_assigned_nodes(&self) -> bool {
+        !self.assigned_nodes.borrow().is_empty()
+    }
+
     /// <https://dom.spec.whatwg.org/#find-flattened-slotables>
     fn find_flattened_slottables(&self, result: &mut RootedVec<Slottable>) {
         // Step 1. Let result be an empty list.
@@ -312,8 +317,12 @@ impl HTMLSlotElement {
         rooted_vec!(let mut slottables);
         self.find_slottables(&mut slottables);
 
-        // Step 2. TODO If slottables and slot’s assigned nodes are not identical,
+        // Step 2. If slottables and slot’s assigned nodes are not identical,
         // then run signal a slot change for slot.
+        let slots_are_identical = self.assigned_nodes.borrow().iter().eq(slottables.iter());
+        if !slots_are_identical {
+            ScriptThread::signal_a_slot_change(self);
+        }
 
         // Step 3. Set slot’s assigned nodes to slottables.
         *self.assigned_nodes.borrow_mut() = slottables.iter().cloned().collect();
diff --git a/components/script/dom/mutationobserver.rs b/components/script/dom/mutationobserver.rs
index e2c86181097..60628764c52 100644
--- a/components/script/dom/mutationobserver.rs
+++ b/components/script/dom/mutationobserver.rs
@@ -15,9 +15,11 @@ use crate::dom::bindings::codegen::Bindings::MutationObserverBinding::{
     MutationCallback, MutationObserverInit,
 };
 use crate::dom::bindings::error::{Error, Fallible};
+use crate::dom::bindings::inheritance::Castable;
 use crate::dom::bindings::reflector::{reflect_dom_object_with_proto, DomObject, Reflector};
 use crate::dom::bindings::root::DomRoot;
 use crate::dom::bindings::str::DOMString;
+use crate::dom::eventtarget::EventTarget;
 use crate::dom::mutationrecord::MutationRecord;
 use crate::dom::node::{Node, ShadowIncluding};
 use crate::dom::window::Window;
@@ -90,35 +92,57 @@ impl MutationObserver {
 
     /// <https://dom.spec.whatwg.org/#queue-a-mutation-observer-compound-microtask>
     pub(crate) fn queue_mutation_observer_microtask() {
-        // Step 1
+        // Step 1. If the surrounding agent’s mutation observer microtask queued is true, then return.
         if ScriptThread::is_mutation_observer_microtask_queued() {
             return;
         }
-        // Step 2
+
+        // Step 2. Set the surrounding agent’s mutation observer microtask queued to true.
         ScriptThread::set_mutation_observer_microtask_queued(true);
-        // Step 3
+
+        // Step 3. Queue a microtask to notify mutation observers.
         ScriptThread::enqueue_microtask(Microtask::NotifyMutationObservers);
     }
 
     /// <https://dom.spec.whatwg.org/#notify-mutation-observers>
-    pub(crate) fn notify_mutation_observers() {
-        // Step 1
+    pub(crate) fn notify_mutation_observers(can_gc: CanGc) {
+        // Step 1. Set the surrounding agent’s mutation observer microtask queued to false.
         ScriptThread::set_mutation_observer_microtask_queued(false);
-        // Step 2
+
+        // Step 2. Let notifySet be a clone of the surrounding agent’s pending mutation observers.
+        // TODO Step 3. Empty the surrounding agent’s pending mutation observers.
         let notify_list = ScriptThread::get_mutation_observers();
-        // TODO: steps 3-4 (slots)
-        // Step 5
+
+        // 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 = ScriptThread::take_signal_slots();
+
+        // Step 6. For each mo of notifySet:
         for mo in &notify_list {
+            // Step 6.1 Let records be a clone of mo’s record queue.
             let queue: Vec<DomRoot<MutationRecord>> = mo.record_queue.borrow().clone();
+
+            // Step 6.2 Empty mo’s record queue.
             mo.record_queue.borrow_mut().clear();
-            // TODO: Step 5.3 Remove all transient registered observers whose observer is mo.
+
+            // TODO Step 6.3 For each node of mo’s node list, remove all transient registered observers
+            // whose observer is mo from node’s registered observer list.
+
+            // Step 6.4 If records is not empty, then invoke mo’s callback with « records,
+            // mo » and "report", and with callback this value mo.
             if !queue.is_empty() {
                 let _ = mo
                     .callback
                     .Call_(&**mo, queue, mo, ExceptionHandling::Report);
             }
         }
-        // TODO: Step 6 (slot signals)
+
+        // Step 6. For each slot of signalSet, fire an event named slotchange,
+        // with its bubbles attribute set to true, at slot.
+        for slot in signal_set {
+            slot.upcast::<EventTarget>()
+                .fire_event(atom!("slotchange"), can_gc);
+        }
     }
 
     /// <https://dom.spec.whatwg.org/#queueing-a-mutation-record>
diff --git a/components/script/dom/node.rs b/components/script/dom/node.rs
index e37e8df1c55..30b15daab05 100644
--- a/components/script/dom/node.rs
+++ b/components/script/dom/node.rs
@@ -2323,6 +2323,48 @@ impl Node {
         let old_next_sibling = node.GetNextSibling();
         // Steps 9-10 are handled in unbind_from_tree.
         parent.remove_child(node, cached_index);
+
+        // Step 12. If node is assigned, then run assign slottables for node’s assigned slot.
+        let assigned_slot = node
+            .downcast::<Element>()
+            .and_then(|e| e.assigned_slot())
+            .or_else(|| {
+                node.downcast::<Text>().and_then(|text| {
+                    text.slottable_data()
+                        .borrow()
+                        .assigned_slot
+                        .as_ref()
+                        .map(Dom::as_rooted)
+                })
+            });
+        if let Some(slot) = assigned_slot {
+            slot.assign_slottables();
+        }
+
+        // Step 13. If parent’s root is a shadow root, and parent is a slot whose assigned nodes is the empty list,
+        // then run signal a slot change for parent.
+        if parent.is_in_a_shadow_tree() {
+            if let Some(slot_element) = parent.downcast::<HTMLSlotElement>() {
+                if !slot_element.has_assigned_nodes() {
+                    ScriptThread::signal_a_slot_change(slot_element);
+                }
+            }
+        }
+
+        // Step 14. If node has an inclusive descendant that is a slot:
+        let has_slot_descendant = node
+            .traverse_preorder(ShadowIncluding::No)
+            .any(|elem| elem.is::<HTMLSlotElement>());
+        if has_slot_descendant {
+            // Step 14.1 Run assign slottables for a tree with parent’s root.
+            parent
+                .GetRootNode(&GetRootNodeOptions::empty())
+                .assign_slottables_for_a_tree();
+
+            // Step 14.2 Run assign slottables for a tree with node.
+            node.assign_slottables_for_a_tree();
+        }
+
         // Step 11. transient registered observers
         // Step 12.
         if let SuppressObserver::Unsuppressed = suppress_observers {
diff --git a/components/script/dom/shadowroot.rs b/components/script/dom/shadowroot.rs
index 46fcb38cf1b..872a3a461d5 100644
--- a/components/script/dom/shadowroot.rs
+++ b/components/script/dom/shadowroot.rs
@@ -325,6 +325,9 @@ impl ShadowRootMethods<crate::DomTypeHolder> for ShadowRoot {
     fn SlotAssignment(&self) -> SlotAssignmentMode {
         self.slot_assignment_mode
     }
+
+    // https://dom.spec.whatwg.org/#dom-shadowroot-onslotchange
+    event_handler!(onslotchange, GetOnslotchange, SetOnslotchange);
 }
 
 impl VirtualMethods for ShadowRoot {
diff --git a/components/script/microtask.rs b/components/script/microtask.rs
index b186d7aa337..94a675d78c9 100644
--- a/components/script/microtask.rs
+++ b/components/script/microtask.rs
@@ -140,7 +140,7 @@ impl MicrotaskQueue {
                         ScriptThread::invoke_backup_element_queue(can_gc);
                     },
                     Microtask::NotifyMutationObservers => {
-                        MutationObserver::notify_mutation_observers();
+                        MutationObserver::notify_mutation_observers(can_gc);
                     },
                     Microtask::ReadableStreamTeeReadRequest(ref task) => {
                         task.microtask_chunk_steps(can_gc)
diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index 13558b81832..c232fb9fe51 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -128,6 +128,7 @@ use crate::dom::element::Element;
 use crate::dom::globalscope::GlobalScope;
 use crate::dom::htmlanchorelement::HTMLAnchorElement;
 use crate::dom::htmliframeelement::HTMLIFrameElement;
+use crate::dom::htmlslotelement::HTMLSlotElement;
 use crate::dom::mutationobserver::MutationObserver;
 use crate::dom::node::{Node, NodeTraits, ShadowIncluding};
 use crate::dom::performanceentry::PerformanceEntry;
@@ -259,6 +260,9 @@ pub struct ScriptThread {
     /// 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>>>,
+
     /// A handle to the WebGL thread
     #[no_trace]
     webgl_chan: Option<WebGLPipeline>,
@@ -508,6 +512,26 @@ impl ScriptThread {
         })
     }
 
+    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()
+                .map(|slot| slot.as_rooted())
+                .collect()
+        })
+    }
+
     pub(crate) fn mark_document_with_no_blocked_loads(doc: &Document) {
         with_script_thread(|script_thread| {
             script_thread
@@ -914,6 +938,7 @@ impl ScriptThread {
             closed_pipelines: DomRefCell::new(HashSet::new()),
             mutation_observer_microtask_queued: Default::default(),
             mutation_observers: Default::default(),
+            signal_slots: Default::default(),
             system_font_service,
             webgl_chan: state.webgl_chan,
             #[cfg(feature = "webxr")]
@@ -3718,6 +3743,15 @@ impl ScriptThread {
             )
         }
     }
+
+    /// <https://dom.spec.whatwg.org/#signal-a-slot-change>
+    pub(crate) fn signal_a_slot_change(slot: &HTMLSlotElement) {
+        // Step 1. Append slot to slot’s relevant agent’s signal slots.
+        ScriptThread::add_signal_slot(slot);
+
+        // Step 2. Queue a mutation observer microtask.
+        MutationObserver::queue_mutation_observer_microtask();
+    }
 }
 
 impl Drop for ScriptThread {