You're right about the geometry thing. Let me rephrase:

my $scr = new MainWindow;
$scr->FullScreen(1);
$scr->configure(-background=>"yellow");

my $canvas = $scr->Canvas(
   -background=>"blue",
   -width=>$scr->width,
   -height=>$scr->height
)->pack( -side=>"top" );
        
my $tree = $canvas->Animation(
   -format => 'GIF', 
   -file => 'tree.gif'
)->start_animation( 20 );
[download]

This yields a blue screen, not a yellow one, with nothing on it. I'm not sure how the pack syntax should be for Animation..? The pod remains silent on the issue...