Was worth a shot, but it looks awful and broken now anyway 🥲:
defaults write -g NSSplitViewItemSidebarDefaultsToFloatingAppearance -bool YES
@fcloth @a @stroughtonsmith yeah. Appears to only work on AppKit / SwiftUI apps not UIKit / SwiftUI Catalyst apps.
@a @stroughtonsmith oh my gosh we need a key for other sidebar types so we can crush the floating appearance in Messages, Music, etc... but I fear it does not exist.