Menu & middle mouse button

Simple script:

local F = cl.Create("Test", 1)
local Item = F.Insert(0)

with Item
    .AddLeft("win.debug(1)")
    .AddRight("win.debug(2)")
    .AddMiddle("win.debug(3)")
    .SetLabel("Test")
endwith

F.Show

quit

So, command list is shown as bar. All three button presses are processed properly.
But if to change bar for menu, that is, change

F.Show

for

F.ShowMenu

then the middle mouse button does nothing and the "3" text is never displayed.
Why? Is it described in manuals?