That behaviour seems to be OS-dependent: I can emulate it using Cygwin; I can't emulate it on Windows XP; I don't have access to Windows 7 to try it there.

In quite a few cases, Tk attempts to retain the look-and-feel of the underlying OS (e.g. window decorations, menus, scrollbars). This may be another of these cases.

Here's a few things you could try:

• Do you get the same behaviour when capturing, and acting upon, the return value. E.g. my $db = $mw->DialogBox(); my $retval = $db->Show(); if ($retval eq "X") { do_X(); }
• Do you get the same behaviour with a global grab, i.e. Show->('-global')
• Does the workaround involving WM_DELETE_WINDOW suggested by fanticla earlier (tk close window) produce different results.
• Check if this behaviour occurs in other non-Tk applications on Windows 7.