LibWeb: Scroll elements into view when they become focused

This makes both user-interactive (tab keys) and programmatic focus
changes scroll the viewport if necessary to reveal the newly focused
element.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -1427,6 +1427,10 @@ void Document::set_focused_element(Element* element)
 
     if (m_layout_root)
         m_layout_root->set_needs_display();
+
+    // Scroll the viewport if necessary to make the newly focused element visible.
+    if (m_focused_element)
+        m_focused_element->scroll_into_view();
 }
 
 void Document::set_active_element(Element* element)

Userland/Libraries/LibWeb/DOM/Element.h

@@ -160,7 +160,7 @@ public:
     WebIDL::ExceptionOr<void> insert_adjacent_text(String const& where, String const& data);
 
     // https://w3c.github.io/csswg-drafts/cssom-view-1/#dom-element-scrollintoview
-    void scroll_into_view(Optional<Variant<bool, ScrollIntoViewOptions>>);
+    void scroll_into_view(Optional<Variant<bool, ScrollIntoViewOptions>> = {});
 
 protected:
     Element(Document&, DOM::QualifiedName);