There is a 2 to 3 pixel gap between the menus. This can be conformed by clicking between the menu dn't show any menuItem of menus i,e either left or right menuItem.

Step to Reproduce:
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1) Run the attached program.
2) Click on any menu, you will see the menu is open ( you can see menuitems).
3) close the menu. Make a note on the line between the menus.
4)Just click between the selected menu one or two pixel between the other menu. you can't any menu poping up.
This is seen atleast from Mustang b40 onwards. I tested this in Dolphine b04.