Help: Disappearing Tk::Adjuster

[cmv]

Wise Monks-

I have need to create 2 widgets - one on top of the other. The bottom one's size needs to optionally be adjustable by the user. I want the adjuster to be in between the 2 widgets. When the whole window is resized, only the top widget should get bigger/smaller.

I've created a test case that does everything I want, but there is a bad side-effect. If you simply run this code, and move the adjuster up high enough - it disappears!

Can you help me find a way to make it stop doing that?

Thanks

-Craig

use strict;
use warnings;

use Tk;
use Tk::Adjuster;

my $t = MainWindow->new;

my $f = $t->Frame(-background=>'green')->
    pack(-expand=>1, -fill=>'both');

my $w1 = $f->Listbox(-background=>'blue')->
    pack(-fill=>'both', -expand=>1, -anchor=>'n');

my $w2 = $f->Listbox(-background=>'yellow', -height=>3)->
    pack(-side=>'bottom', -fill=>'both', -expand=>1, -anchor=>'n');

my $adj1 = $f->Adjuster();
$adj1->packAfter($w2, -side=>'bottom');

MainLoop;