popover menu: Disappear on focus out

We try to keep focus inside by wrapping arrow keynav.
But focus can still escape. If it does, we want to
disappear, like any good menu should.

gtk/gtkpopovermenu.c

@@ -26,6 +26,8 @@
 #include "gtkmenutracker.h"
 #include "gtkpopoverprivate.h"
 #include "gtkwidgetprivate.h"
+#include "gtkeventcontrollerkey.h"
+#include "gtkmain.h"
 
 
 /**
@@ -137,11 +139,26 @@ visible_submenu_changed (GObject        *object,
   g_object_notify (G_OBJECT (popover), "visible-submenu");
 }
 
+static void
+focus_out (GtkEventController *controller,
+           GdkCrossingMode     mode,
+           GdkNotifyType       detail,
+           GtkPopover         *popover)
+{
+  gboolean contains_focus;
+
+  g_object_get (controller, "contains-focus", &contains_focus, NULL);
+
+  if (!contains_focus)
+    gtk_popover_popdown (popover);
+}
+
 static void
 gtk_popover_menu_init (GtkPopoverMenu *popover)
 {
   GtkWidget *stack;
   GtkStyleContext *style_context;
+  GtkEventController *controller;
 
   stack = gtk_stack_new ();
   gtk_stack_set_vhomogeneous (GTK_STACK (stack), FALSE);
@@ -153,6 +170,10 @@ gtk_popover_menu_init (GtkPopoverMenu *popover)
 
   style_context = gtk_widget_get_style_context (GTK_WIDGET (popover));
   gtk_style_context_add_class (style_context, GTK_STYLE_CLASS_MENU);
+
+  controller = gtk_event_controller_key_new ();
+  g_signal_connect (controller, "focus-out", G_CALLBACK (focus_out), popover);
+  gtk_widget_add_controller (GTK_WIDGET (popover), controller);
 }
 
 static void