LibWeb: Add HTMLOptionsCollection selected index property

Tests/LibWeb/Text/expected/select.txt

@@ -11,3 +11,4 @@
 11. 999
 12. 10
 13. 10
+14. "5 5"

Tests/LibWeb/Text/input/select.html

@@ -135,5 +135,16 @@
             select.length = -10;
             return select.length;
         });
+
+        // 14. OptionsCollection selectedIndex is the same as select selectedIndex
+        testPart(() => {
+            const select = document.createElement('select');
+            for (let i = 0; i < 10; i++) {
+                const option = document.createElement('option');
+                if (i == 5) option.selected = true;
+                select.appendChild(option);
+            }
+            return `${select.options.selectedIndex} ${select.selectedIndex}`;
+        });
     });
 </script>

Userland/Libraries/LibWeb/HTML/HTMLOptionsCollection.cpp

@@ -111,4 +111,17 @@ WebIDL::ExceptionOr<void> HTMLOptionsCollection::add(HTMLOptionOrOptGroupElement
     return {};
 }
 
+// https://html.spec.whatwg.org/#dom-htmloptionscollection-selectedindex
+WebIDL::Long HTMLOptionsCollection::selected_index() const
+{
+    // The selectedIndex IDL attribute must act like the identically named attribute
+    // on the select element on which the HTMLOptionsCollection is rooted.
+    return verify_cast<HTMLSelectElement>(*root()).selected_index();
+}
+
+void HTMLOptionsCollection::set_selected_index(WebIDL::Long index)
+{
+    verify_cast<HTMLSelectElement>(*root()).set_selected_index(index);
+}
+
 }

Userland/Libraries/LibWeb/HTML/HTMLOptionsCollection.h

@@ -28,6 +28,9 @@ public:
 
     WebIDL::ExceptionOr<void> add(HTMLOptionOrOptGroupElement element, Optional<HTMLElementOrElementIndex> before = {});
 
+    WebIDL::Long selected_index() const;
+    void set_selected_index(WebIDL::Long);
+
 private:
     HTMLOptionsCollection(DOM::ParentNode& root, Function<bool(DOM::Element const&)> filter);
 

Userland/Libraries/LibWeb/HTML/HTMLOptionsCollection.idl

@@ -9,5 +9,5 @@ interface HTMLOptionsCollection : HTMLCollection {
     // [CEReactions] setter undefined (unsigned long index, HTMLOptionElement? option);
     [CEReactions] undefined add((HTMLOptionElement or HTMLOptGroupElement) element, optional (HTMLElement or long)? before = null);
     // [CEReactions] undefined remove(long index);
-    // attribute long selectedIndex;
+    attribute long selectedIndex;
 };