All the new and cool WWDC2020 stuff for BigSur is based on AppKit. And there is currently no SwiftUI implementation for this part. But I'm also interested in this and how to achieve this with SwiftUI. I'm a really big fan of SwiftUI and how it works but for now it seems that SwiftUI is more a iOS/iPadOS thing then a macOS thing.
But maybe this one will do the trick. - Haven't tested this yet...
NSApp.keyWindow?.toolbar?.insertItem(withItemIdentifier: .sidebarTrackingSeparator, at: 1)
MarkusBux
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