we create focusLine set to undefined I think, then later set it to null when we mean not to scroll. But here we check for "falsiness" which I think for JS includes zero, so we couldn't scroll to line 0. Not that we really want to anyway, maybe.

Still I'd tend to create focusLine with `ko.observable(null)` and then make this `line !== null` perhaps? or `typeof(line) === 'number'` maybe?

Sorry I didn't notice this earlier on the line number PR.