# A right-click in the viewer should bring up the popup menu.
$viewer->bind("<3>", [\&post_popup_menu]);

sub post_popup_menu {
  my $w = shift; # no idea how this works.
  my $X = $w->XEvent->X;
  my $Y = $w->XEvent->Y;
  $viewer_menu->Post($X, $Y);
}