That's a two step process currently when you are just going through the comments you click "root" on any sub-root-comment and then "next" on the root comment. Next will always take you to the next sibling comment (called a sibling, because it's a tree data-structure basically and sibling comes from a family tree. In fact where I said sub-root-comment I could have said descendants or children.)

EDIT: I just realized that the first child of the root doesn't have the "root" button so you have to click "parent" on it and then "next".