so say you have a scroll control that is 10x10 with a swatch inside that is also 10x10
you add a new line to whatever is on the swatch, so it needs to be 4 pixels larger and scroll to the bottom. Make it 4 pixels taller and then make it 4 pixels higher
%swatch.resize(0, -4, 10, 14);
(assuming you have some magical custom scroll control with no border)
should do the trick.
or if you want something that will always work when you extend the swatch by %a pixels
%swatch.resize(getword(%swatch.getposition(), 0), getword(%swatch.getposition(), 1)-%a, getword(%swatch.getextent(), 0), getword(%swatch.getextent(), 1)+%a);