Auto merge of #16883 - jdm:mutationobserver, r=jdm

Mutation Observer API

Rebased from #16668.

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix (partially) #6633
- [X] There are tests for these changes

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9 files changed, 370 insertions, 23 deletions

diff --git a/components/script/dom/attr.rs b/components/script/dom/attr.rs
index 3c775ffdf57..af5f0107ae4 100644
--- a/components/script/dom/attr.rs
+++ b/components/script/dom/attr.rs
@@ -10,6 +10,8 @@ use dom::bindings::js::{LayoutJS, MutNullableJS, Root, RootedReference};
 use dom::bindings::reflector::{Reflector, reflect_dom_object};
 use dom::bindings::str::DOMString;
 use dom::element::{AttributeMutation, Element};
+use dom::mutationobserver::{Mutation, MutationObserver};
+use dom::node::Node;
 use dom::virtualmethods::vtable_for;
 use dom::window::Window;
 use dom_struct::dom_struct;
@@ -170,6 +172,12 @@ impl AttrMethods for Attr {
 
 impl Attr {
     pub fn set_value(&self, mut value: AttrValue, owner: &Element) {
+        let name = self.local_name().clone();
+        let namespace = self.namespace().clone();
+        let old_value = DOMString::from(&**self.value());
+        let mutation = Mutation::Attribute { name, namespace, old_value };
+        MutationObserver::queue_a_mutation_record(owner.upcast::<Node>(), mutation);
+
         assert!(Some(owner) == self.owner().r());
         owner.will_mutate_attr(self);
         self.swap_value(&mut value);
diff --git a/components/script/dom/element.rs b/components/script/dom/element.rs
index 91ea1435302..011b87aabf1 100644
--- a/components/script/dom/element.rs
+++ b/components/script/dom/element.rs
@@ -63,6 +63,7 @@ use dom::htmltablerowelement::{HTMLTableRowElement, HTMLTableRowElementLayoutHel
 use dom::htmltablesectionelement::{HTMLTableSectionElement, HTMLTableSectionElementLayoutHelpers};
 use dom::htmltemplateelement::HTMLTemplateElement;
 use dom::htmltextareaelement::{HTMLTextAreaElement, LayoutHTMLTextAreaElementHelpers};
+use dom::mutationobserver::{Mutation, MutationObserver};
 use dom::namednodemap::NamedNodeMap;
 use dom::node::{CLICK_IN_PROGRESS, ChildrenMutation, LayoutNodeHelpers, Node};
 use dom::node::{NodeDamage, SEQUENTIALLY_FOCUSABLE, UnbindContext};
@@ -1003,6 +1004,12 @@ impl Element {
     }
 
     pub fn push_attribute(&self, attr: &Attr) {
+        let name = attr.local_name().clone();
+        let namespace = attr.namespace().clone();
+        let old_value = DOMString::from(&**attr.value());
+        let mutation = Mutation::Attribute { name, namespace, old_value };
+        MutationObserver::queue_a_mutation_record(&self.node, mutation);
+
         assert!(attr.GetOwnerElement().r() == Some(self));
         self.will_mutate_attr(attr);
         self.attrs.borrow_mut().push(JS::from_ref(attr));
@@ -1125,13 +1132,18 @@ impl Element {
     }
 
     fn remove_first_matching_attribute<F>(&self, find: F) -> Option<Root<Attr>>
-        where F: Fn(&Attr) -> bool
-    {
+        where F: Fn(&Attr) -> bool {
         let idx = self.attrs.borrow().iter().position(|attr| find(&attr));
-
         idx.map(|idx| {
             let attr = Root::from_ref(&*(*self.attrs.borrow())[idx]);
             self.will_mutate_attr(&attr);
+
+            let name = attr.local_name().clone();
+            let namespace = attr.namespace().clone();
+            let old_value = DOMString::from(&**attr.value());
+            let mutation = Mutation::Attribute { name, namespace, old_value, };
+            MutationObserver::queue_a_mutation_record(&self.node, mutation);
+
             self.attrs.borrow_mut().remove(idx);
             attr.set_owner(None);
             if attr.namespace() == &ns!() {
diff --git a/components/script/dom/mutationobserver.rs b/components/script/dom/mutationobserver.rs
index 4dc745c1ce8..b174ae4d11f 100644
--- a/components/script/dom/mutationobserver.rs
+++ b/components/script/dom/mutationobserver.rs
@@ -2,13 +2,22 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+use dom::bindings::callback::ExceptionHandling;
+use dom::bindings::cell::DOMRefCell;
 use dom::bindings::codegen::Bindings::MutationObserverBinding;
 use dom::bindings::codegen::Bindings::MutationObserverBinding::MutationCallback;
-use dom::bindings::error::Fallible;
-use dom::bindings::js::Root;
+use dom::bindings::codegen::Bindings::MutationObserverBinding::MutationObserverBinding::MutationObserverMethods;
+use dom::bindings::codegen::Bindings::MutationObserverBinding::MutationObserverInit;
+use dom::bindings::error::{Error, Fallible};
+use dom::bindings::js::{JS, Root};
 use dom::bindings::reflector::{Reflector, reflect_dom_object};
+use dom::bindings::str::DOMString;
+use dom::mutationrecord::MutationRecord;
+use dom::node::Node;
 use dom::window::Window;
 use dom_struct::dom_struct;
+use html5ever::{Namespace, LocalName};
+use microtask::Microtask;
 use script_thread::ScriptThread;
 use std::rc::Rc;
 
@@ -17,6 +26,29 @@ pub struct MutationObserver {
     reflector_: Reflector,
     #[ignore_heap_size_of = "can't measure Rc values"]
     callback: Rc<MutationCallback>,
+    record_queue: DOMRefCell<Vec<Root<MutationRecord>>>,
+}
+
+#[derive(Clone)]
+pub enum Mutation {
+    Attribute { name: LocalName, namespace: Namespace, old_value: DOMString }
+}
+
+#[derive(HeapSizeOf, JSTraceable)]
+pub struct RegisteredObserver {
+    observer: Root<MutationObserver>,
+    options: ObserverOptions,
+}
+
+#[derive(HeapSizeOf, JSTraceable)]
+pub struct ObserverOptions {
+    attribute_old_value: bool,
+    attributes: bool,
+    character_data: bool,
+    character_data_old_value: bool,
+    child_list: bool,
+    subtree: bool,
+    attribute_filter: Vec<DOMString>,
 }
 
 impl MutationObserver {
@@ -29,6 +61,7 @@ impl MutationObserver {
         MutationObserver {
             reflector_: Reflector::new(),
             callback: callback,
+            record_queue: DOMRefCell::new(vec![]),
         }
     }
 
@@ -37,4 +70,185 @@ impl MutationObserver {
         ScriptThread::add_mutation_observer(&*observer);
         Ok(observer)
     }
+
+    /// https://dom.spec.whatwg.org/#queue-a-mutation-observer-compound-microtask
+    pub fn queue_mutation_observer_compound_microtask() {
+        // Step 1
+        if ScriptThread::is_mutation_observer_compound_microtask_queued() {
+            return;
+        }
+        // Step 2
+        ScriptThread::set_mutation_observer_compound_microtask_queued(true);
+        // Step 3
+        ScriptThread::enqueue_microtask(Microtask::NotifyMutationObservers);
+    }
+
+    /// https://dom.spec.whatwg.org/#notify-mutation-observers
+    pub fn notify_mutation_observers() {
+        // Step 1
+        ScriptThread::set_mutation_observer_compound_microtask_queued(false);
+        // Step 2
+        let notify_list = ScriptThread::get_mutation_observers();
+        // TODO: steps 3-4 (slots)
+        // Step 5
+        for mo in &notify_list {
+            let queue: Vec<Root<MutationRecord>> = mo.record_queue.borrow().clone();
+            mo.record_queue.borrow_mut().clear();
+            // TODO: Step 5.3 Remove all transient registered observers whose observer is mo.
+            if !queue.is_empty() {
+                let _ = mo.callback.Call_(&**mo, queue, &**mo, ExceptionHandling::Report);
+            }
+        }
+        // TODO: Step 6 (slot signals)
+    }
+
+    /// https://dom.spec.whatwg.org/#queueing-a-mutation-record
+    pub fn queue_a_mutation_record(target: &Node, attr_type: Mutation) {
+        // Step 1
+        let mut interestedObservers: Vec<(Root<MutationObserver>, Option<DOMString>)> = vec![];
+        // Step 2 & 3
+        for node in target.inclusive_ancestors() {
+            for registered in &*node.registered_mutation_observers() {
+                if &*node != target && !registered.options.subtree {
+                    continue;
+                }
+
+                match attr_type {
+                    Mutation::Attribute { ref name, ref namespace, ref old_value } => {
+                        // Step 3.1
+                        if !registered.options.attributes {
+                            continue;
+                        }
+                        if !registered.options.attribute_filter.is_empty() {
+                            if *namespace != ns!() {
+                                continue;
+                            }
+                            if registered.options.attribute_filter.iter()
+                                .find(|s| &**s == &**name).is_some() {
+                                continue;
+                            }
+                        }
+                        // Step 3.1.2
+                        let paired_string = if registered.options.attribute_old_value {
+                            Some(old_value.clone())
+                        } else {
+                            None
+                        };
+                        // Step 3.1.1
+                        let idx = interestedObservers.iter().position(|&(ref o, _)|
+                            &**o as *const _ == &*registered.observer as *const _);
+                        if let Some(idx) = idx {
+                            interestedObservers[idx].1 = paired_string;
+                        } else {
+                            interestedObservers.push((Root::from_ref(&*registered.observer),
+                                                      paired_string));
+                        }
+                    }
+                }
+            }
+        }
+
+        // Step 4
+        for &(ref observer, ref paired_string) in &interestedObservers {
+            // Steps 4.1-4.7
+            let record = match attr_type {
+                Mutation::Attribute { ref name, ref namespace, .. } => {
+                    let namespace = if *namespace != ns!() {
+                        Some(namespace)
+                    } else {
+                        None
+                    };
+                    MutationRecord::attribute_mutated(target, name, namespace, paired_string.clone())
+                }
+            };
+            // Step 4.8
+            observer.record_queue.borrow_mut().push(record);
+        }
+
+        // Step 5
+        MutationObserver::queue_mutation_observer_compound_microtask();
+    }
+
+}
+
+impl MutationObserverMethods for MutationObserver {
+    /// https://dom.spec.whatwg.org/#dom-mutationobserver-observe
+    fn Observe(&self, target: &Node, options: &MutationObserverInit) -> Fallible<()> {
+        let attribute_filter = options.attributeFilter.clone().unwrap_or(vec![]);
+        let attribute_old_value = options.attributeOldValue.unwrap_or(false);
+        let mut attributes = options.attributes.unwrap_or(false);
+        let mut character_data = options.characterData.unwrap_or(false);
+        let character_data_old_value = options.characterDataOldValue.unwrap_or(false);
+        let child_list = options.childList;
+        let subtree = options.subtree;
+
+        // Step 1
+        if (options.attributeOldValue.is_some() || options.attributeFilter.is_some()) &&
+            options.attributes.is_none() {
+            attributes = true;
+        }
+
+        // Step 2
+        if options.characterDataOldValue.is_some() && options.characterData.is_none() {
+            character_data = true;
+        }
+
+        // Step 3
+        if !child_list && !attributes && !character_data {
+            return Err(Error::Type("One of childList, attributes, or characterData must be true".into()));
+        }
+
+        // Step 4
+        if attribute_old_value && !attributes {
+            return Err(Error::Type("attributeOldValue is true but attributes is false".into()));
+        }
+
+        // Step 5
+        if options.attributeFilter.is_some() && !attributes {
+            return Err(Error::Type("attributeFilter is present but attributes is false".into()));
+        }
+
+        // Step 6
+        if character_data_old_value && !character_data {
+            return Err(Error::Type("characterDataOldValue is true but characterData is false".into()));
+        }
+
+        // Step 7
+        let add_new_observer = {
+            let mut replaced = false;
+            for registered in &mut *target.registered_mutation_observers() {
+                if &*registered.observer as *const MutationObserver != self as *const MutationObserver {
+                    continue;
+                }
+                // TODO: remove matching transient registered observers
+                registered.options.attribute_old_value = attribute_old_value;
+                registered.options.attributes = attributes;
+                registered.options.character_data = character_data;
+                registered.options.character_data_old_value = character_data_old_value;
+                registered.options.child_list = child_list;
+                registered.options.subtree = subtree;
+                registered.options.attribute_filter = attribute_filter.clone();
+                replaced = true;
+            }
+            !replaced
+        };
+
+        // Step 8
+        if add_new_observer {
+            target.registered_mutation_observers().push(RegisteredObserver {
+                observer: Root::from_ref(self),
+                options: ObserverOptions {
+                    attributes,
+                    attribute_old_value,
+                    character_data,
+                    character_data_old_value,
+                    subtree,
+                    attribute_filter,
+                    child_list
+                },
+            });
+        }
+
+        Ok(())
+    }
 }
diff --git a/components/script/dom/mutationrecord.rs b/components/script/dom/mutationrecord.rs
index c39d61ef18a..439f4ae02b5 100644
--- a/components/script/dom/mutationrecord.rs
+++ b/components/script/dom/mutationrecord.rs
@@ -2,22 +2,54 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+use dom::bindings::codegen::Bindings::MutationRecordBinding::MutationRecordBinding;
 use dom::bindings::codegen::Bindings::MutationRecordBinding::MutationRecordBinding::MutationRecordMethods;
 use dom::bindings::js::{JS, Root};
-use dom::bindings::reflector::Reflector;
+use dom::bindings::reflector::{Reflector, reflect_dom_object};
 use dom::bindings::str::DOMString;
-use dom::node::Node;
+use dom::node::{Node, window_from_node};
+use dom::nodelist::NodeList;
 use dom_struct::dom_struct;
+use html5ever::{LocalName, Namespace};
 
 #[dom_struct]
 pub struct MutationRecord {
     reflector_: Reflector,
-
-    //property for record type
     record_type: DOMString,
-
-    //property for target node
     target: JS<Node>,
+    attribute_name: Option<DOMString>,
+    attribute_namespace: Option<DOMString>,
+    old_value: Option<DOMString>,
+}
+
+impl MutationRecord {
+    #[allow(unrooted_must_root)]
+    pub fn attribute_mutated(target: &Node,
+                             attribute_name: &LocalName,
+                             attribute_namespace: Option<&Namespace>,
+                             old_value: Option<DOMString>) -> Root<MutationRecord> {
+        let record = box MutationRecord::new_inherited("attributes",
+                                                       target,
+                                                       Some(DOMString::from(&**attribute_name)),
+                                                       attribute_namespace.map(|n| DOMString::from(&**n)),
+                                                       old_value);
+        reflect_dom_object(record, &*window_from_node(target), MutationRecordBinding::Wrap)
+    }
+
+    fn new_inherited(record_type: &str,
+                     target: &Node,
+                     attribute_name: Option<DOMString>,
+                     attribute_namespace: Option<DOMString>,
+                     old_value: Option<DOMString>) -> MutationRecord {
+        MutationRecord {
+            reflector_: Reflector::new(),
+            record_type: DOMString::from(record_type),
+            target: JS::from_ref(target),
+            attribute_name: attribute_name,
+            attribute_namespace: attribute_namespace,
+            old_value: old_value,
+        }
+    }
 }
 
 impl MutationRecordMethods for MutationRecord {
@@ -28,7 +60,44 @@ impl MutationRecordMethods for MutationRecord {
 
     // https://dom.spec.whatwg.org/#dom-mutationrecord-target
     fn Target(&self) -> Root<Node> {
-        return Root::from_ref(&*self.target);
+        Root::from_ref(&*self.target)
+    }
+
+    // https://dom.spec.whatwg.org/#dom-mutationrecord-attributename
+    fn GetAttributeName(&self) -> Option<DOMString> {
+        self.attribute_name.clone()
+    }
+
+    // https://dom.spec.whatwg.org/#dom-mutationrecord-attributenamespace
+    fn GetAttributeNamespace(&self) -> Option<DOMString> {
+        self.attribute_namespace.clone()
+    }
+
+    // https://dom.spec.whatwg.org/#dom-mutationrecord-oldvalue
+    fn GetOldValue(&self) -> Option<DOMString> {
+        self.old_value.clone()
+    }
+
+    // https://dom.spec.whatwg.org/#dom-mutationrecord-addednodes
+    fn AddedNodes(&self) -> Root<NodeList> {
+        let window = window_from_node(&*self.target);
+        NodeList::empty(&window)
+    }
+
+    // https://dom.spec.whatwg.org/#dom-mutationrecord-removednodes
+    fn RemovedNodes(&self) -> Root<NodeList> {
+        let window = window_from_node(&*self.target);
+        NodeList::empty(&window)
+    }
+
+    // https://dom.spec.whatwg.org/#dom-mutationrecord-previoussibling
+    fn GetPreviousSibling(&self) -> Option<Root<Node>> {
+        None
+    }
+
+    // https://dom.spec.whatwg.org/#dom-mutationrecord-previoussibling
+    fn GetNextSibling(&self) -> Option<Root<Node>> {
+        None
     }
 
 }
diff --git a/components/script/dom/node.rs b/components/script/dom/node.rs
index bec7a34d63f..8c8831801ef 100644
--- a/components/script/dom/node.rs
+++ b/components/script/dom/node.rs
@@ -7,6 +7,7 @@
 use app_units::Au;
 use devtools_traits::NodeInfo;
 use document_loader::DocumentLoader;
+use dom::bindings::cell::DOMRefCell;
 use dom::bindings::codegen::Bindings::CharacterDataBinding::CharacterDataMethods;
 use dom::bindings::codegen::Bindings::DocumentBinding::DocumentMethods;
 use dom::bindings::codegen::Bindings::ElementBinding::ElementMethods;
@@ -46,6 +47,7 @@ use dom::htmllinkelement::HTMLLinkElement;
 use dom::htmlmetaelement::HTMLMetaElement;
 use dom::htmlstyleelement::HTMLStyleElement;
 use dom::htmltextareaelement::{HTMLTextAreaElement, LayoutHTMLTextAreaElementHelpers};
+use dom::mutationobserver::RegisteredObserver;
 use dom::nodelist::NodeList;
 use dom::processinginstruction::ProcessingInstruction;
 use dom::range::WeakRangeVec;
@@ -72,7 +74,7 @@ use selectors::matching::matches_selector_list;
 use selectors::parser::SelectorList;
 use servo_url::ServoUrl;
 use std::borrow::ToOwned;
-use std::cell::{Cell, UnsafeCell};
+use std::cell::{Cell, UnsafeCell, RefMut};
 use std::cmp::max;
 use std::default::Default;
 use std::iter;
@@ -138,6 +140,9 @@ pub struct Node {
     /// node is finalized.
     style_and_layout_data: Cell<Option<OpaqueStyleAndLayoutData>>,
 
+    /// Registered observers for this node.
+    mutation_observers: DOMRefCell<Vec<RegisteredObserver>>,
+
     unique_id: UniqueId,
 }
 
@@ -363,6 +368,11 @@ impl Node {
         }
     }
 
+    /// Return all registered mutation observers for this node.
+    pub fn registered_mutation_observers(&self) -> RefMut<Vec<RegisteredObserver>> {
+         self.mutation_observers.borrow_mut()
+    }
+
     /// Dumps the subtree rooted at this node, for debugging.
     pub fn dump(&self) {
         self.dump_indent(0);
@@ -1411,6 +1421,8 @@ impl Node {
 
             style_and_layout_data: Cell::new(None),
 
+            mutation_observers: Default::default(),
+
             unique_id: UniqueId::new(),
         }
     }
diff --git a/components/script/dom/webidls/MutationObserver.webidl b/components/script/dom/webidls/MutationObserver.webidl
index dbcfa945d4a..738c711d8e7 100644
--- a/components/script/dom/webidls/MutationObserver.webidl
+++ b/components/script/dom/webidls/MutationObserver.webidl
@@ -9,7 +9,8 @@
 // https://dom.spec.whatwg.org/#mutationobserver
 [Pref="dom.mutation_observer.enabled", Constructor(MutationCallback callback)]
 interface MutationObserver {
-    //void observe(Node target, optional MutationObserverInit options);
+    [Throws]
+    void observe(Node target, optional MutationObserverInit options);
     //void disconnect();
     //sequence<MutationRecord> takeRecords();
 };
diff --git a/components/script/dom/webidls/MutationRecord.webidl b/components/script/dom/webidls/MutationRecord.webidl
index 286801bf2ab..1cd364091fc 100644
--- a/components/script/dom/webidls/MutationRecord.webidl
+++ b/components/script/dom/webidls/MutationRecord.webidl
@@ -12,13 +12,13 @@ interface MutationRecord {
     readonly attribute DOMString type;
     [SameObject]
     readonly attribute Node target;
-    //[SameObject]
-    //readonly attribute NodeList addedNodes;
-    //[SameObject]
-    //readonly attribute NodeList removedNodes;
-    //readonly attribute Node? previousSibling;
-    //readonly attribute Node? nextSibling;
-    //readonly attribute DOMString? attributeName;
-    //readonly attribute DOMString? attributeNamespace;
-    //readonly attribute DOMString? oldValue;
+    [SameObject]
+    readonly attribute NodeList addedNodes;
+    [SameObject]
+    readonly attribute NodeList removedNodes;
+    readonly attribute Node? previousSibling;
+    readonly attribute Node? nextSibling;
+    readonly attribute DOMString? attributeName;
+    readonly attribute DOMString? attributeNamespace;
+    readonly attribute DOMString? oldValue;
 };
diff --git a/components/script/microtask.rs b/components/script/microtask.rs
index da04d1ed6ab..7fadf111ddb 100644
--- a/components/script/microtask.rs
+++ b/components/script/microtask.rs
@@ -11,6 +11,7 @@ use dom::bindings::cell::DOMRefCell;
 use dom::bindings::codegen::Bindings::PromiseBinding::PromiseJobCallback;
 use dom::bindings::js::Root;
 use dom::globalscope::GlobalScope;
+use dom::mutationobserver::MutationObserver;
 use msg::constellation_msg::PipelineId;
 use std::cell::Cell;
 use std::mem;
@@ -28,6 +29,7 @@ pub struct MicrotaskQueue {
 #[derive(JSTraceable, HeapSizeOf)]
 pub enum Microtask {
     Promise(EnqueuedPromiseCallback),
+    NotifyMutationObservers,
 }
 
 /// A promise callback scheduled to run during the next microtask checkpoint (#4283).
@@ -71,6 +73,9 @@ impl MicrotaskQueue {
                             let _ = job.callback.Call_(&*target, ExceptionHandling::Report);
                         }
                     }
+                    Microtask::NotifyMutationObservers => {
+                        MutationObserver::notify_mutation_observers();
+                    }
                 }
             }
         }
diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index 9ab6fd301bc..a8b20b6cf98 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -481,6 +481,9 @@ pub struct ScriptThread {
 
     microtask_queue: MicrotaskQueue,
 
+    /// Microtask Queue for adding support for mutation observer microtasks
+    mutation_observer_compound_microtask_queued: Cell<bool>,
+
     /// The unit of related similar-origin browsing contexts' list of MutationObserver objects
     mutation_observers: DOMRefCell<Vec<JS<MutationObserver>>>,
 
@@ -589,6 +592,20 @@ impl ScriptThread {
         })
     }
 
+    pub fn set_mutation_observer_compound_microtask_queued(value: bool) {
+        SCRIPT_THREAD_ROOT.with(|root| {
+            let script_thread = unsafe { &*root.get().unwrap() };
+            script_thread.mutation_observer_compound_microtask_queued.set(value);
+        })
+    }
+
+    pub fn is_mutation_observer_compound_microtask_queued() -> bool {
+        SCRIPT_THREAD_ROOT.with(|root| {
+            let script_thread = unsafe { &*root.get().unwrap() };
+            return script_thread.mutation_observer_compound_microtask_queued.get();
+        })
+    }
+
     pub fn add_mutation_observer(observer: &MutationObserver) {
         SCRIPT_THREAD_ROOT.with(|root| {
             let script_thread = unsafe { &*root.get().unwrap() };
@@ -598,6 +615,13 @@ impl ScriptThread {
         })
     }
 
+    pub fn get_mutation_observers() -> Vec<Root<MutationObserver>> {
+        SCRIPT_THREAD_ROOT.with(|root| {
+            let script_thread = unsafe { &*root.get().unwrap() };
+            script_thread.mutation_observers.borrow().iter().map(|o| Root::from_ref(&**o)).collect()
+        })
+    }
+
     pub fn mark_document_with_no_blocked_loads(doc: &Document) {
         SCRIPT_THREAD_ROOT.with(|root| {
             let script_thread = unsafe { &*root.get().unwrap() };
@@ -750,6 +774,8 @@ impl ScriptThread {
 
             microtask_queue: MicrotaskQueue::default(),
 
+            mutation_observer_compound_microtask_queued: Default::default(),
+
             mutation_observers: Default::default(),
 
             layout_to_constellation_chan: state.layout_to_constellation_chan,