Interestingly, if you create a Listbox that's not scrolled, it works correctly.

I wasn't able to make it change the height even with

sub change_height {
    my ($widget, $height) = @_;
    print "Configuring $widget\n";
    eval { $widget->configure(-height => $height) };
    for my $subwidget ($widget->Subwidget) {
        change_height($subwidget, $height) if $subwidget;
    }
}
         
 $mw->Button(-text => 'Change Height',
       -font => $myFont,
       -command => sub { change_height($lb, 5); }, 
       )->pack();
[download]

I have no idea what to try next.