I have a lot of top level sources now in my sources view. I'll often want to collapse them all and the only way I've found to do it is to select All and then do "collapse completely." It would sort of be nice if there was a way to do a collapse completely that includes all top level folders?
Collapse all top-level folders
You're correct that collapsing everything requires selecting all the top-level folders. The problem is there's no concept of a single top-level folder (or root) that one can select and collapse. A solution would be to add some sort of command to "close all folders in the notebook".
The phrasing would need to be improved, but I'd be willing to do it if you could request the feature be added.
Alfonso