racket/gui gtk: make `on-demand' for menus work better

There appears to be no way to make `on-demand' work right for the
Unity window manager's global menu bar, since there's no notificiation
when the menu bar is clicked. We approximate the correct behavior
by calling `on-demand' when a containing frame loses the keyboard
focus, which might be because the menu bar was clicked; that may be
too late (because the menu has already been shown), but it should
work most of the time.

Closes PR 13347.

Relevant to PR 13395, but DrRacket will have to change to work
around the remaining limitations of `on-demand'.

collects/mred/private/wx/gtk/frame.rkt

@@ -404,9 +404,14 @@
 				  (set! reported-activate on?)
 				  (on-activate on?)))))))
 
+    (define treat-focus-out-as-menu-click? #f)
+    (define/public (treat-focus-out-as-menu-click)
+      (set! treat-focus-out-as-menu-click? #t))
+
     (define focus-here? #f)
     (define/override (on-focus? on?)
-      (unless on? (on-menu-click))
+      (when (and (not on?) treat-focus-out-as-menu-click?)
+        (on-menu-click))
       (on-focus-child on?)
       (cond
        [on?

collects/mred/private/wx/gtk/menu-bar.rkt

@@ -26,6 +26,9 @@
 
 (define-gtk gtk_widget_set_usize (_fun _GtkWidget _int _int -> _void))
 
+(define-gtk ubuntu_menu_proxy_get _fpointer
+  #:fail (lambda () #f))
+
 (define (fixup-mnemonic title)
   (mnemonic-string (regexp-replace #rx"\t.*$" title "")))
 
@@ -92,6 +95,8 @@
 
   (define/public (set-top-window top)
     (set! top-wx top)
+    (when ubuntu_menu_proxy_get
+      (send top treat-focus-out-as-menu-click))
     (install-widget-parent top)
     (fix-menu-height))
 

collects/scribblings/gui/labelled-menu-item-intf.scrbl

@@ -72,6 +72,7 @@ Normally called when the user clicks on the menu bar containing the
  item (before the user sees any menu items), just before the popup
  menu containing the item is popped up, or just before inspecting the
  menu bar containing the item for a shortcut key binding.
+ See @xmethod[menu-item-container<%> on-demand] for further details.
 
 A @xmethod[menu-item-container<%> on-demand] method can be overridden
 in such a way that the container does not call the

collects/scribblings/gui/menu-item-container-intf.scrbl

@@ -21,11 +21,12 @@ Returns a list of the items in the menu, popup menu, or menu bar. The
 @methspec{
 
 Called when the user clicks on the container as a menu bar (before the
- user sees any menu items), just before the container as a popup menu
+ user sees any menu items, except with Unity's global menu bar as
+ noted below), just before the container as a popup menu
  is popped up, or just before inspecting the menu bar containing the
  item for a shortcut key binding.
 
-If the container is not a menu bar or a popup menu, this method is
+If the container is not a @tech{menu bar} or a @tech{popup menu}, this method is
  normally called via the @method[menu-item-container<%> on-demand]
  method of the container's owning menu bar or popup menu, because the
  default implementation of the method chains to the
@@ -34,6 +35,17 @@ If the container is not a menu bar or a popup menu, this method is
  it does not call the @method[labelled-menu-item<%> on-demand] method
  of its items.
 
+On Unix with the Unity window manager using the global menu bar (which
+ is the default on Ubuntu), @racket[racket/gui/base] receives no
+ notification when the user clicks the menu bar. To approximate
+ @method[menu-item-container<%> on-demand] triggered by user clicks of
+ the menu bar, @method[menu-item-container<%> on-demand] is called for
+ a @tech{menu bar} whenever its @racket[frame%] object loses the
+ keyboard focus. Beware that if keyboard focus was lost because a menu
+ was clicked, then items added to the clicked menu during an
+ @method[menu-item-container<%> on-demand] invocation may not appear
+ for the user.
+
 }
 @methimpl{