LibGUI: Let Widgets track their focus proxy delegators

And update them on relevant events.

Userland/Libraries/LibGUI/Widget.cpp

@@ -633,6 +633,11 @@ void Widget::update()
     if (rect().is_empty())
         return;
     update(rect());
+
+    for (auto& it : m_focus_delegators) {
+        if (!it.is_null() && !it->rect().is_empty())
+            it->update(it->rect());
+    }
 }
 
 void Widget::update(Gfx::IntRect const& rect)
@@ -726,10 +731,28 @@ void Widget::set_focus_proxy(Widget* proxy)
 {
     if (m_focus_proxy == proxy)
         return;
-
+    if (proxy)
+        proxy->add_focus_delegator(this);
+    else if (m_focus_proxy)
+        m_focus_proxy->remove_focus_delegator(this);
     m_focus_proxy = proxy;
 }
 
+void Widget::add_focus_delegator(Widget* delegator)
+{
+    m_focus_delegators.remove_all_matching([&](auto& entry) {
+        return entry.is_null() || entry == delegator;
+    });
+    m_focus_delegators.append(delegator);
+}
+
+void Widget::remove_focus_delegator(Widget* delegator)
+{
+    m_focus_delegators.remove_first_matching([&](auto& entry) {
+        return entry == delegator;
+    });
+}
+
 FocusPolicy Widget::focus_policy() const
 {
     if (m_focus_proxy)

Userland/Libraries/LibGUI/Widget.h

@@ -226,6 +226,9 @@ public:
     Widget const* focus_proxy() const { return m_focus_proxy; }
     void set_focus_proxy(Widget*);
 
+    Vector<WeakPtr<Widget>>& focus_delegators() { return m_focus_delegators; }
+    Vector<WeakPtr<Widget>> const& focus_delegators() const { return m_focus_delegators; }
+
     void set_focus_policy(FocusPolicy policy);
     FocusPolicy focus_policy() const;
 
@@ -406,6 +409,9 @@ protected:
 
     void show_or_hide_tooltip();
 
+    void add_focus_delegator(Widget*);
+    void remove_focus_delegator(Widget*);
+
 private:
     virtual bool is_widget() const final { return true; }
 
@@ -453,6 +459,7 @@ private:
     String m_title { String::empty() };
 
     WeakPtr<Widget> m_focus_proxy;
+    Vector<WeakPtr<Widget>> m_focus_delegators;
     FocusPolicy m_focus_policy { FocusPolicy::NoFocus };
 
     AK::Variant<Gfx::StandardCursor, NonnullRefPtr<Gfx::Bitmap>> m_override_cursor { Gfx::StandardCursor::None };