Implement shadow dom slots (#35013)

* Implement slot-related algorithms

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

* Hook up slot elements to DOM creation logic

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

* Set a slot assignment mode for servo-internal shadow roots

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

* Assign slots when a slottable's slot attribute changes

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

* Properly compute slot name

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

* ./mach test-tidy

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

* Update <slot> name when name attribute changes

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

* Implement fast path for Node::assign_slottables_for_a_tree

assign_slottables_for_a_tree traverses all descendants of the node
and is potentially very expensive. If the node is not a shadow root
then assigning slottables to it won't have any effect, so we
take a fast path out.

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

* Move slottable data into ElementRareData

This shrinks all element descendants back to their
original size.

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

* Address review comments

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

* Update WPT expectations

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

---------

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

3 files changed, 28 insertions, 3 deletions

diff --git a/components/script/dom/webidls/Element.webidl b/components/script/dom/webidls/Element.webidl
index c7884925541..60a6db4e4e1 100644
--- a/components/script/dom/webidls/Element.webidl
+++ b/components/script/dom/webidls/Element.webidl
@@ -90,7 +90,7 @@ interface Element : Node {
 dictionary ShadowRootInit {
   required ShadowRootMode mode;
   // boolean delegatesFocus = false;
-  // SlotAssignmentMode slotAssignment = "named";
+  SlotAssignmentMode slotAssignment = "named";
   boolean clonable = false;
   // boolean serializable = false;
 };
diff --git a/components/script/dom/webidls/HTMLSlotElement.webidl b/components/script/dom/webidls/HTMLSlotElement.webidl
new file mode 100644
index 00000000000..bec872ab1fc
--- /dev/null
+++ b/components/script/dom/webidls/HTMLSlotElement.webidl
@@ -0,0 +1,25 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+// https://html.spec.whatwg.org/multipage/#the-slot-element
+[Exposed=Window]
+interface HTMLSlotElement : HTMLElement {
+  [HTMLConstructor] constructor();
+
+  [CEReactions] attribute DOMString name;
+  sequence<Node> assignedNodes(optional AssignedNodesOptions options = {});
+  sequence<Element> assignedElements(optional AssignedNodesOptions options = {});
+  undefined assign((Element or Text)... nodes);
+};
+
+dictionary AssignedNodesOptions {
+  boolean flatten = false;
+};
+
+// https://dom.spec.whatwg.org/#mixin-slotable
+interface mixin Slottable {
+  readonly attribute HTMLSlotElement? assignedSlot;
+};
+Element includes Slottable;
+Text includes Slottable;
diff --git a/components/script/dom/webidls/ShadowRoot.webidl b/components/script/dom/webidls/ShadowRoot.webidl
index 40301f48517..444dd53d22c 100644
--- a/components/script/dom/webidls/ShadowRoot.webidl
+++ b/components/script/dom/webidls/ShadowRoot.webidl
@@ -10,7 +10,7 @@
 interface ShadowRoot : DocumentFragment {
   readonly attribute ShadowRootMode mode;
   // readonly attribute boolean delegatesFocus;
-  // readonly attribute SlotAssignmentMode slotAssignment;
+  readonly attribute SlotAssignmentMode slotAssignment;
   readonly attribute boolean clonable;
   // readonly attribute boolean serializable;
   readonly attribute Element host;
@@ -19,7 +19,7 @@ interface ShadowRoot : DocumentFragment {
 
 
 enum ShadowRootMode { "open", "closed"};
-// enum SlotAssignmentMode { "manual", "named" };
+enum SlotAssignmentMode { "manual", "named" };
 
 ShadowRoot includes DocumentOrShadowRoot;