Interplay between cell focus and collapsing

[kieferrm]

Testing #141222

I'd consider collapsing and expanding a way to focus on what's matters while exploring a notebook. When exploring cell focus is less important to me. I move my move cursor while reading and want to hide what I just looked at. But to do so, I first have to click to move the cell focus, then I have to reposition my mouse to get the hover feedback of the focus indicator, and then I have to click again. Just like folding indicators in the notebook and text editors, I'd expect a single click to do the trick for me.

[roblourens]

We should show a gray version of that indicator on hover