As you adapt your app for iPhone Duo, you may find yourself considering putting a NavigationSplitView inside a TabView. It’s true that UIKit recommends against nesting a split view controller within another container in most cases[^1] and that many Apple apps treat the sidebar and tab bar as alternate presentations of the same navigation. But there isn’t a blanket design rule against list-detail navigation within a tab. The distinction is what each view does: - A TabView with the .sidebarAdaptable style lets people switch between top-level areas of your app, presented as tabs or a sidebar. - A NavigationSplitView lets people navigate through a hierarchy, where selecting something in one column controls what appears in the next. Those can be complementary. The pitfall to avoid is having multiple sidebars doing the same job. In fact, the HIG even suggests using a sidebar within a tab on visionOS when your app’s hierarchy is deep[^2]. Just make sure the sidebar selection doesn’t change which tab is open! If tabs organize your app’s sections and a split view gives each section a useful list-detail layout, I say go for it. A new form factor like the Duo is a good time to revisit old assumptions. But given UIKit’s historical guidance, be mindful you may encounter some rough edges. This would be a good question for the UI frameworks team at a future iPhone Duo Apple Group Lab. [^1]: developer.apple.com/document… [^2]: developer.apple.com/design/h……