I'm going to weigh in and support "rotatable skillbars". shift-uparrow/shift-downarrow to rotate through skillbars.

Points in it's favor:
1. You get to choose what you put on which skillbar, so you can group skills in a fashion that makes sense to you
2. Low screen real-estate requirements- you can put some clickable arrows to change the active skillbar, and that's all the additional screen space you take.
3. It's very easy to demonstrate this to new players during the tutorial or whenever, and it's simple to understand.


This arrangement with per-skill family pop-ups is a massive over-design, solving a non-problem, because most/no characters are not going to have all or most of the skills from all the trees.