LibWeb: Look for labeled control in DOM tree instead of layout tree

...because "change" event should be dispatched on control even if it
has "display: none" style.

This change fixes selection in labels dropdown on GitHub's "new issue"
page.

Userland/Libraries/LibWeb/HTML/HTMLLabelElement.cpp

@@ -30,4 +30,39 @@ JS::GCPtr<Layout::Node> HTMLLabelElement::create_layout_node(NonnullRefPtr<CSS::
     return heap().allocate_without_realm<Layout::Label>(document(), this, move(style));
 }
 
+// https://html.spec.whatwg.org/multipage/forms.html#labeled-control
+JS::GCPtr<HTMLElement> HTMLLabelElement::control() const
+{
+    JS::GCPtr<HTMLElement> control;
+
+    // The for attribute may be specified to indicate a form control with which the caption is
+    // to be associated. If the attribute is specified, the attribute's value must be the ID of
+    // a labelable element in the same tree as the label element. If the attribute is specified
+    // and there is an element in the tree whose ID is equal to the value of the for attribute,
+    // and the first such element in tree order is a labelable element, then that element is the
+    // label element's labeled control.
+    if (for_().has_value()) {
+        for_each_in_inclusive_subtree_of_type<HTMLElement>([&](auto& element) {
+            if (element.id() == *for_() && element.is_labelable()) {
+                control = &const_cast<HTMLElement&>(element);
+                return IterationDecision::Break;
+            }
+            return IterationDecision::Continue;
+        });
+        return control;
+    }
+
+    // If the for attribute is not specified, but the label element has a labelable element descendant,
+    // then the first such descendant in tree order is the label element's labeled control.
+    for_each_in_subtree_of_type<HTMLElement>([&](auto& element) {
+        if (element.is_labelable()) {
+            control = &const_cast<HTMLElement&>(element);
+            return IterationDecision::Break;
+        }
+        return IterationDecision::Continue;
+    });
+
+    return control;
+}
+
 }