LibWeb: Add CustomStateSet IDL type

Author: https://github.com/AtkinsSJ
Commit: e63d81b36e
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5302
Reviewed-by: https://github.com/tcl3 ✅

Libraries/LibWeb/HTML/CustomElements/CustomStateSet.cpp

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibGC/CellAllocator.h>
+#include <LibJS/Runtime/PrimitiveString.h>
+#include <LibJS/Runtime/Realm.h>
+#include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/DOM/Element.h>
+#include <LibWeb/HTML/CustomElements/CustomStateSet.h>
+
+namespace Web::HTML {
+
+GC_DEFINE_ALLOCATOR(CustomStateSet);
+
+GC::Ref<CustomStateSet> CustomStateSet::create(JS::Realm& realm, GC::Ref<DOM::Element> element)
+{
+    return realm.create<CustomStateSet>(realm, element);
+}
+
+CustomStateSet::CustomStateSet(JS::Realm& realm, GC::Ref<DOM::Element> element)
+    : Bindings::PlatformObject(realm)
+    , m_set_entries(JS::Set::create(realm))
+    , m_element(element)
+{
+}
+
+void CustomStateSet::initialize(JS::Realm& realm)
+{
+    WEB_SET_PROTOTYPE_FOR_INTERFACE(CustomStateSet);
+    Base::initialize(realm);
+}
+
+void CustomStateSet::visit_edges(Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(m_set_entries);
+    visitor.visit(m_element);
+}
+
+bool CustomStateSet::has_state(FlyString const& state) const
+{
+    return m_set_entries->set_has(JS::PrimitiveString::create(realm().vm(), state));
+}
+
+void CustomStateSet::on_set_modified_from_js(Badge<Bindings::CustomStateSetPrototype>)
+{
+    m_element->invalidate_style(DOM::StyleInvalidationReason::CustomStateSetChange);
+}
+
+}

Libraries/LibWeb/HTML/CustomElements/CustomStateSet.h

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Set.h>
+#include <LibJS/Runtime/SetIterator.h>
+#include <LibWeb/Bindings/CustomStateSetPrototype.h>
+#include <LibWeb/Bindings/PlatformObject.h>
+
+namespace Web::HTML {
+
+// https://html.spec.whatwg.org/multipage/custom-elements.html#customstateset
+class CustomStateSet final : public Bindings::PlatformObject {
+    WEB_PLATFORM_OBJECT(CustomStateSet, Bindings::PlatformObject);
+    GC_DECLARE_ALLOCATOR(CustomStateSet);
+
+public:
+    [[nodiscard]] static GC::Ref<CustomStateSet> create(JS::Realm&, GC::Ref<DOM::Element>);
+    virtual ~CustomStateSet() override = default;
+
+    GC::Ref<JS::Set> set_entries() const { return m_set_entries; }
+    bool has_state(FlyString const&) const;
+
+    void on_set_modified_from_js(Badge<Bindings::CustomStateSetPrototype>);
+
+private:
+    CustomStateSet(JS::Realm&, GC::Ref<DOM::Element>);
+
+    virtual void initialize(JS::Realm&) override;
+    virtual void visit_edges(Cell::Visitor&) override;
+
+    GC::Ref<JS::Set> m_set_entries;
+    GC::Ref<DOM::Element> m_element;
+};
+
+}

Libraries/LibWeb/HTML/CustomElements/CustomStateSet.idl

@@ -0,0 +1,5 @@
+// https://html.spec.whatwg.org/multipage/custom-elements.html#customstateset
+[Exposed=Window]
+interface CustomStateSet {
+    setlike<DOMString>;
+};