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

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.