Spin button inside a menu item

[Nix <nix go-nix ca>]

Hi !

I would like to create a popup menu that looks like this:

...
menu items
...
Insert <spinbutton> elements
...
menu items
...

I can build it easily enough, but I cannot convince the menu widget to
give input focus to the spin button contained within it.

I would like the following behaviour:
- When I move the mouse over the spin button menu item, it gets 
  hilighted (normal menu behaviour).
- When the mouse enters the spin button, the hilight disappears, and the
  spin button gets input focus.  This allows me to click inside the 
  entry, type a number, click the up and down arrows, etc., without the
  menu collapsing. At this point, the popup menu thinks the mouse is 
  not hovering over it at all - this is why none of the items are 
  hilighted.
- When the mouse leaves the spin button, the usual menu selection 
  resumes.

Here's the source code building the menu, a window to click to make it
pop up, and some futile efforts at handing focus to the spin button:

http://qcadesigner.ca/nix/gtk-menu.c

TIA for your advice !