LibGUI: Add Widget::has_focus_within()

This returns true if the widget has focus, or if one of its descendant
widgets does. Use this in StackWidget and TabWidget.

This also fixes HackStudio crashing on startup in StackWidget, due to
running before the window has a focused widget.

Libraries/LibGUI/StackWidget.cpp

@@ -43,7 +43,7 @@ void StackWidget::set_active_widget(Widget* widget)
     if (widget == m_active_widget)
         return;
 
-    bool active_widget_had_focus = m_active_widget && window() && (window()->focused_widget() == m_active_widget || m_active_widget->is_ancestor_of(*window()->focused_widget()));
+    bool active_widget_had_focus = m_active_widget && m_active_widget->has_focus_within();
 
     if (m_active_widget)
         m_active_widget->set_visible(false);

Libraries/LibGUI/TabWidget.cpp

@@ -88,12 +88,13 @@ void TabWidget::update_focus_policy()
         policy = FocusPolicy::NoFocus;
     set_focus_policy(policy);
 }
+
 void TabWidget::set_active_widget(Widget* widget)
 {
     if (widget == m_active_widget)
         return;
 
-    bool active_widget_had_focus = m_active_widget && window() && (window()->focused_widget() == m_active_widget || m_active_widget->is_ancestor_of(*window()->focused_widget()));
+    bool active_widget_had_focus = m_active_widget && m_active_widget->has_focus_within();
 
     if (m_active_widget)
         m_active_widget->set_visible(false);

Libraries/LibGUI/Widget.cpp

@@ -989,4 +989,16 @@ Widget* Widget::find_descendant_by_name(const String& name)
     });
     return found_widget;
 }
+
+bool Widget::has_focus_within() const
+{
+    auto* window = this->window();
+    if (!window)
+        return false;
+    if (!window->focused_widget())
+        return false;
+    auto& effective_focus_widget = focus_proxy() ? *focus_proxy() : *this;
+    return window->focused_widget() == &effective_focus_widget || is_ancestor_of(*window->focused_widget());
+}
+
 }

Libraries/LibGUI/Widget.h

@@ -165,6 +165,9 @@ public:
     bool is_focused() const;
     void set_focus(bool, FocusSource = FocusSource::Programmatic);
 
+    // Returns true if this widget or one of its descendants is focused.
+    bool has_focus_within() const;
+
     Widget* focus_proxy() { return m_focus_proxy; }
     const Widget* focus_proxy() const { return m_focus_proxy; }
     void set_focus_proxy(Widget*);