LibWeb: Allow focusing individual (focusable) elements with Tab key

You can now cycle through focusable elements (currently only hyperlinks
are focusable) with the Tab key.

The focus outline is rendered in a new FocusOutline paint phase.

Libraries/LibWeb/DOM/Document.cpp

@@ -508,4 +508,18 @@ bool Document::is_editable() const
     return m_editable;
 }
 
+void Document::set_focused_element(Element* element)
+{
+    if (m_focused_element == element)
+        return;
+
+    if (element)
+        m_focused_element = element->make_weak_ptr();
+    else
+        m_focused_element = nullptr;
+
+    if (m_layout_root)
+        m_layout_root->set_needs_display();
+}
+
 }

Libraries/LibWeb/DOM/Document.h

@@ -161,6 +161,11 @@ public:
     void set_editable(bool editable) { m_editable = editable; }
     virtual bool is_editable() const final;
 
+    Element* focused_element() { return m_focused_element; }
+    const Element* focused_element() const { return m_focused_element; }
+
+    void set_focused_element(Element*);
+
 private:
     virtual RefPtr<LayoutNode> create_layout_node(const CSS::StyleProperties* parent_style) override;
 
@@ -191,6 +196,8 @@ private:
 
     QuirksMode m_quirks_mode { QuirksMode::No };
     bool m_editable { false };
+
+    WeakPtr<Element> m_focused_element;
 };
 
 }

Libraries/LibWeb/DOM/Element.cpp

@@ -295,4 +295,9 @@ String Element::inner_html() const
     return builder.to_string();
 }
 
+bool Element::is_focused() const
+{
+    return document().focused_element() == this;
+}
+
 }

Libraries/LibWeb/DOM/Element.h

@@ -87,6 +87,9 @@ public:
     String inner_html() const;
     void set_inner_html(StringView);
 
+    bool is_focused() const;
+    virtual bool is_focusable() const { return false; }
+
 protected:
     RefPtr<LayoutNode> create_layout_node(const CSS::StyleProperties* parent_style) override;