This is a feature idea to auto-fold/unfold notebooks when they are selected in the Sidebar.
When selecting a new notebook, it would be nice to have the option to have that note automatically unfold subnotebooks when selected, and a sub-option to autoclose other notebooks. This would be especially usable when you have many Notebook folders in the root/sub-notebooks.
Current behavior is that you can either select a notebook. or the little fold/unfold arrow next to it to unfold the same notebook. I think this is redundant, as I cannot think of many situations where I would want to select a notebook and not open it to see whats inside it.
If this suggestion was implemented, I could still be able to manipulate a Notebook (New, Delete, etc) by right-clicking the notebook, same as the current behaviour.
A remaining use-case that could potentially have a problem with the suggestion behaviour would be drag and drop of notes and notebooks from other notebooks, but that could be remedied by creating a short waiting period before the folder unfolds on hover, to allow better control of such simple file movements (In MacOS you can set the time for the Finder to wait before folders open).
Note, I think this should be an optional setting, not a standard replacing the old behaviour.