List view: Add and handle a max-width for deep nesting cases

[jasmussen]

Extracting from #35605: if you add deeply nested blocks to your canvas, the list view can expand beyond its boundaries. Video:

CleanShot.2021-10-13.at.10.56.00.mp4

There are some explorations in the ticket linked around how to handle that, including flattening the hierarchy as it bumps against the right edge, and adding a horizontal scrollbar when needed.

In the case of adding a scrollbar, the ellipsis on the right can potentially become fixed position, as suggested here:

It may be fine to start just with adding the scrollbar and seeing how often we encounter the overflow, and how pressing the handling of it is.

[jasmussen]

Closing this as fixed by the addition of the scrollbar.