tkwait variable name
tkwait visibility name
tkwait window name
While the tkwait command is waiting it processes events in the normal fashion, so the application will continue to respond to user interactions. If an event handler invokes tkwait again, the nested call to tkwait must complete before the outer call can complete.
http://www.purl.org/tcl/home/man/tcl8.4/TkCmd/tkwait.html
"Anyway, it's a little bit confusing that tkwait is 'scope-proof'. ;)" Another reminder to read the man page carefully: "If the first argument is variable (or any abbreviation of it) then the second argument is the name of a global variable and the command waits for that variable to be modified."
See also:
Tk syntax help - Arts and Crafts of Tcl-Tk Programming - Category Command