Go really doesn't like us doing anything inheritance-y: it does not support open recursion meaning
it's really hard to re-use code. As such, here we're falling back to conditional logic.
This fixes an issue where our ListContextTrait was calling FocusLine which was intended to be
overridden by ViewportListContextTrait, but the subclassed function wasn't being called. I'm
not actually sure how this went wrong given that it was working fine in the past, but at any rate,
the new code is easy to follow.
