Bug 1712140 - Part 5: Block Declarative ShadowDOM behind feature pref. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D194846

dom/webidl/Document.webidl

@@ -151,6 +151,7 @@ interface Document : Node {
 
 // https://html.spec.whatwg.org/multipage/dom.html#the-document-object
 partial interface Document {
+  [Pref="dom.webcomponents.shadowdom.declarative.enabled"]
   static Document parseHTMLUnsafe(DOMString html);
 
   [PutForwards=href, LegacyUnforgeable] readonly attribute Location? location;

dom/webidl/Element.webidl

@@ -276,6 +276,7 @@ dictionary ShadowRootInit {
   required ShadowRootMode mode;
   boolean delegatesFocus = false;
   SlotAssignmentMode slotAssignment = "named";
+  [Pref="dom.webcomponents.shadowdom.declarative.enabled"]
   boolean clonable = false;
 };
 
@@ -406,5 +407,7 @@ partial interface Element {
 };
 
 partial interface Element {
+  // https://html.spec.whatwg.org/#dom-element-sethtmlunsafe
+  [Pref="dom.webcomponents.shadowdom.declarative.enabled"]
   undefined setHTMLUnsafe(DOMString html);
 };

dom/webidl/HTMLTemplateElement.webidl

@@ -14,6 +14,8 @@ interface HTMLTemplateElement : HTMLElement {
   [HTMLConstructor] constructor();
 
   readonly attribute DocumentFragment content;
-  [CEReactions] attribute DOMString shadowRootMode;
-  [CEReactions] attribute boolean shadowRootDelegatesFocus;
+  [CEReactions, Pref="dom.webcomponents.shadowdom.declarative.enabled"]
+  attribute DOMString shadowRootMode;
+  [CEReactions, Pref="dom.webcomponents.shadowdom.declarative.enabled"]
+  attribute boolean shadowRootDelegatesFocus;
 };

dom/webidl/ShadowRoot.webidl

@@ -57,6 +57,8 @@ interface ShadowRoot : DocumentFragment
 };
 
 partial interface ShadowRoot {
+  // https://html.spec.whatwg.org/#dom-shadowroot-sethtmlunsafe
+  [Pref="dom.webcomponents.shadowdom.declarative.enabled"]
   undefined setHTMLUnsafe(DOMString html);
 };
 

layout/build/nsContentDLF.cpp

@@ -314,7 +314,8 @@ nsresult nsContentDLF::CreateDocument(
   nsCOMPtr<nsIDocumentViewer> viewer = NS_NewDocumentViewer();
 
   doc->SetContainer(static_cast<nsDocShell*>(aContainer));
-  doc->SetAllowDeclarativeShadowRoots(true);
+  doc->SetAllowDeclarativeShadowRoots(
+      mozilla::StaticPrefs::dom_webcomponents_shadowdom_declarative_enabled());
 
   // Initialize the document to begin loading the data.  An
   // nsIStreamListener connected to the parser is returned in

modules/libpref/init/StaticPrefList.yaml

@@ -4528,6 +4528,12 @@
   value: false
   mirror: always
 
+# Is support for Declarative ShadowDOM enabled?
+- name: dom.webcomponents.shadowdom.declarative.enabled
+  type: bool
+  value: @IS_NIGHTLY_BUILD@
+  mirror: always
+
 # Is support for the Web GPU API enabled?
 - name: dom.webgpu.enabled
   type: RelaxedAtomicBool