A recurring question on comp.lang.tcl is, ``How can I center a toplevel window on the screen?''

The usual answer is to use the [update] command to wait for the
window to be configured, and then use the [[wm]] command to adjust
its geometry.

There is an alternative way to do it that doesn't involve the many
pitfalls of [update]: bind to the <Configure> event and use that to
notify the process that the window has been configured and it's
time to compute the geometry.  The code below gives the idea:
----
    # KBK - 12 February 2001

    proc make_the_window {} {

        # Create the toplevel window

        toplevel .t
        grid [label .t.l -text "This window\nshould be centered."]
        grid [button .t.d -text Dismiss -command {destroy .t}]

        # Set things up to position the window when it is
        # configured.

        bind .t <Configure> {center_the_toplevel %W}

        return

    }
    
    proc center_the_toplevel { w } {

        # Callback on the <Configure> event for a toplevel
        # that should be centered on the screen

        # Make sure that we aren't configuring a child window

        if { [string equal $w [winfo toplevel $w]] } {

            # Calculate the desired geometry

            set width [winfo reqwidth $w]
            set height [winfo reqheight $w]    
            set x [expr { ( [winfo vrootwidth  $w] - $width  ) / 2 }]
            set y [expr { ( [winfo vrootheight $w] - $height ) / 2 }]

            # Hand the geometry off to the window manager

            wm geometry $w ${width}x${height}+${x}+${y}

            # Unbind <Configure> so that this procedure is
            # not called again when the window manager finishes
            # centering the window

            bind $w <Configure> {}

        }
        
        return
    }
      
    grid [button .b -text "Test" -command make_the_window] \
        [button .q -text "Quit" -command exit]