Re: mystery #1: I think you've got the sense of -scrollregion and -width/-height confused. Try this:

my $c = $mw->Scrolled('Canvas',
-width => 500,
-height => 500,
-background =>'blue',
-scrollregion => [ 0, 0, 200, 200 ]
);
[download]

And try resizing the window to something large (larger than 500x500) before pressing the '+' button.