$canvas->set_center_scroll_region(FALSE);

##</code><code>##

$canvas->set_size(400, 100);
$canvas->set_center_scroll_region(TRUE);
$frame->add($canvas);

##</code><code>##

$canvas->set_center_scroll_region(TRUE);
$canvas->set_size(400, 100);
$frame->add($canvas);