The 8.6 [tk busy] command does not cause menu's attached to toplevels via configure -menu (menu widget name) to become "busy" when it makes the rest of the toplevel busy.
I.e., run the following in an 8.6 interpreter:
button .b -text "Button\n.b" -command [ puts stderr "Button .b" ]
.mbar.file add command -label "Open" -command [ list puts stderr "Open" ]
.mbar.file add command -label "Close" -command [ list puts stderr "Close" ]
.mbar add cascade -menu .mbar.file -label File
. configure -menu .mbar
tk busy .
Not that the button is "busy" and unavailable, but the menu attached to . via the -menu option is still available, and can be accessed.
The busy.n man page makes no mention of what should be the expected behavior of performing [tk busy .toplevel] when .toplevel has a -menu attached. I was expecting that the attached menu would also be made busy along with the rest of the toplevel window.
I tested this with 8.6b2 under Linux and with an 8.6b1 starkit under windows. Both Linux and Windows exhibited the bug.