I'm on macOS 14.1 Beta (23B5067a) / Xcode 15.1 beta (15C5028h) but I was seeing this issue before I updated to the betas. It's possible (not confirmed) that it may have been related to having nested List { Section(title:isExpanded:) } content.

Thanks! That's helpful.

Oh, nice – I missed it because I was looking for the chevron on the left side and missed that it's only visible when the cursor is on the line. This should work for me, thank you! Any ideas why what I was trying to do wasn't working?

Not yet but it's something that I may look at in the longer term. For the time being, I'm using a top-level NavigationSplitView and doing something more primitive in place of the inner NavigationSplitView that lets me make progress with my current setup. (Or course, then I just ran into a bug involving DisclosureGroup so ...) Have you tried mixing NavigationSplitView and NavigationStack (i.e. using the NavigationStack inside the detail: of the NavigationSplitView)?