I had the same problem, if you have the NavigationSplitview in a NavigationStack, then you should remove it and put the NavigationStack in the detail area.

The spacing issue is back since 18.4. Any suggestions?

Hi Becca, thank you very much! I hope the problem will be solved soon, otherwise I see no other way to rebuild the NavigationSplitView. The sidebar of the TabView is not sufficient for my needs, as I have to use a highly customized view here.