LibGUI: Allow widget sibling navigation with arrow keys

There's no spatial navigation here, Left/Up moves to the previous
sibling in the tab order, while Right/Down moves to the next.

The arrow keys keep focus within the same parent widget, unlike the tab
key which cycles through all focusable widgets in the window.

This makes GUI::MessageBox feel a bit nicer since you can now arrow
between the Yes/No/Cancel buttons. :^)

Libraries/LibGUI/Widget.cpp

@@ -445,13 +445,25 @@ void Widget::hide_event(HideEvent&)
 
 void Widget::keydown_event(KeyEvent& event)
 {
-    if (!event.alt() && !event.ctrl() && !event.logo() && event.key() == KeyCode::Key_Tab) {
-        if (event.shift())
-            focus_previous_widget(FocusSource::Keyboard);
-        else
-            focus_next_widget(FocusSource::Keyboard);
-        event.accept();
-        return;
+    if (!event.alt() && !event.ctrl() && !event.logo()) {
+        if (event.key() == KeyCode::Key_Tab) {
+            if (event.shift())
+                focus_previous_widget(FocusSource::Keyboard, false);
+            else
+                focus_next_widget(FocusSource::Keyboard, false);
+            event.accept();
+            return;
+        }
+        if (!event.shift() && (event.key() == KeyCode::Key_Left || event.key() == KeyCode::Key_Up)) {
+            focus_previous_widget(FocusSource::Keyboard, true);
+            event.accept();
+            return;
+        }
+        if (!event.shift() && (event.key() == KeyCode::Key_Right || event.key() == KeyCode::Key_Down)) {
+            focus_next_widget(FocusSource::Keyboard, true);
+            event.accept();
+            return;
+        }
     }
     event.ignore();
 }
@@ -822,9 +834,11 @@ void Widget::set_updates_enabled(bool enabled)
         update();
 }
 
-void Widget::focus_previous_widget(FocusSource source)
+void Widget::focus_previous_widget(FocusSource source, bool siblings_only)
 {
     auto focusable_widgets = window()->focusable_widgets(source);
+    if (siblings_only)
+        focusable_widgets.remove_all_matching([this](auto& entry) { return entry->parent() != parent(); });
     for (int i = focusable_widgets.size() - 1; i >= 0; --i) {
         if (focusable_widgets[i] != this)
             continue;
@@ -835,9 +849,11 @@ void Widget::focus_previous_widget(FocusSource source)
     }
 }
 
-void Widget::focus_next_widget(FocusSource source)
+void Widget::focus_next_widget(FocusSource source, bool siblings_only)
 {
     auto focusable_widgets = window()->focusable_widgets(source);
+    if (siblings_only)
+        focusable_widgets.remove_all_matching([this](auto& entry) { return entry->parent() != parent(); });
     for (size_t i = 0; i < focusable_widgets.size(); ++i) {
         if (focusable_widgets[i] != this)
             continue;

Libraries/LibGUI/Widget.h

@@ -341,8 +341,8 @@ private:
     void handle_mouseup_event(MouseEvent&);
     void handle_enter_event(Core::Event&);
     void handle_leave_event(Core::Event&);
-    void focus_previous_widget(FocusSource);
-    void focus_next_widget(FocusSource);
+    void focus_previous_widget(FocusSource, bool siblings_only);
+    void focus_next_widget(FocusSource, bool siblings_only);
 
     void show_tooltip();