LibWeb: Implement automatic slottable assignment

This implements automatic slottable assignment by way of hooking into
the element attribute change steps. When the `name` attribute of a slot
or the `slot` attribute of a slottable changes, assignment is performed.

Userland/Libraries/LibWeb/DOM/Element.cpp

@@ -71,6 +71,38 @@ Element::Element(Document& document, DOM::QualifiedName qualified_name)
     , m_qualified_name(move(qualified_name))
 {
     make_html_uppercased_qualified_name();
+
+    // https://dom.spec.whatwg.org/#ref-for-concept-element-attributes-change-ext①
+    add_attribute_change_steps([this](auto const& local_name, auto const& old_value, auto const& value, auto const& namespace_) {
+        // 1. If localName is slot and namespace is null, then:
+        if (local_name == HTML::AttributeNames::slot && namespace_.is_null()) {
+            // 1. If value is oldValue, then return.
+            if (value == old_value)
+                return;
+
+            // 2. If value is null and oldValue is the empty string, then return.
+            if (value.is_null() && old_value == DeprecatedString::empty())
+                return;
+
+            // 3. If value is the empty string and oldValue is null, then return.
+            if (value == DeprecatedString::empty() && old_value.is_null())
+                return;
+
+            // 4. If value is null or the empty string, then set element’s name to the empty string.
+            if (value.is_empty())
+                set_slottable_name({});
+            // 5. Otherwise, set element’s name to value.
+            else
+                set_slottable_name(MUST(String::from_deprecated_string(value)));
+
+            // 6. If element is assigned, then run assign slottables for element’s assigned slot.
+            if (auto assigned_slot = assigned_slot_internal())
+                assign_slottables(*assigned_slot);
+
+            // 7. Run assign a slot for element.
+            assign_a_slot(JS::NonnullGCPtr { *this });
+        }
+    });
 }
 
 Element::~Element() = default;

Userland/Libraries/LibWeb/DOM/Slottable.cpp

@@ -84,8 +84,19 @@ JS::GCPtr<HTML::HTMLSlotElement> find_a_slot(Slottable const& slottable, OpenFla
         return slot;
     }
 
-    // FIXME: 6. Return the first slot in tree order in shadow’s descendants whose name is slottable’s name, if any; otherwise null.
+    // 6. Return the first slot in tree order in shadow’s descendants whose name is slottable’s name, if any; otherwise null.
-    return nullptr;
+    auto const& slottable_name = slottable.visit([](auto const& node) { return node->slottable_name(); });
+    JS::GCPtr<HTML::HTMLSlotElement> slot;
+
+    shadow->for_each_in_subtree_of_type<HTML::HTMLSlotElement>([&](auto& child) {
+        if (child.slot_name() != slottable_name)
+            return IterationDecision::Continue;
+
+        slot = child;
+        return IterationDecision::Break;
+    });
+
+    return slot;
 }
 
 // https://dom.spec.whatwg.org/#find-slotables
@@ -118,8 +129,19 @@ Vector<Slottable> find_slottables(JS::NonnullGCPtr<HTML::HTMLSlotElement> slot)
     }
     // 6. Otherwise, for each slottable child slottable of host, in tree order:
     else {
-        // FIXME: 1. Let foundSlot be the result of finding a slot given slottable.
+        host->for_each_child([&](auto& node) {
-        // FIXME: 2. If foundSlot is slot, then append slottable to result.
+            if (!node.is_slottable())
+                return;
+
+            auto slottable = node.as_slottable();
+
+            // 1. Let foundSlot be the result of finding a slot given slottable.
+            auto found_slot = find_a_slot(slottable);
+
+            // 2. If foundSlot is slot, then append slottable to result.
+            if (found_slot == slot)
+                result.append(move(slottable));
+        });
     }
 
     // 7. Return result.

Userland/Libraries/LibWeb/HTML/HTMLSlotElement.cpp

@@ -15,6 +15,33 @@ namespace Web::HTML {
 HTMLSlotElement::HTMLSlotElement(DOM::Document& document, DOM::QualifiedName qualified_name)
     : HTMLElement(document, move(qualified_name))
 {
+    // https://dom.spec.whatwg.org/#ref-for-concept-element-attributes-change-ext
+    add_attribute_change_steps([this](auto const& local_name, auto const& old_value, auto const& value, auto const& namespace_) {
+        // 1. If element is a slot, localName is name, and namespace is null, then:
+        if (local_name == AttributeNames::name && namespace_.is_null()) {
+            // 1. If value is oldValue, then return.
+            if (value == old_value)
+                return;
+
+            // 2. If value is null and oldValue is the empty string, then return.
+            if (value.is_null() && old_value == DeprecatedString::empty())
+                return;
+
+            // 3. If value is the empty string and oldValue is null, then return.
+            if (value == DeprecatedString::empty() && old_value.is_null())
+                return;
+
+            // 4. If value is null or the empty string, then set element’s name to the empty string.
+            if (value.is_empty())
+                set_slot_name({});
+            // 5. Otherwise, set element’s name to value.
+            else
+                set_slot_name(MUST(String::from_deprecated_string(value)));
+
+            // 6. Run assign slottables for a tree with element’s root.
+            DOM::assign_slottables_for_a_tree(root());
+        }
+    });
 }
 
 HTMLSlotElement::~HTMLSlotElement() = default;