NSToolbarFlexibleSpaceItem is constraint to NSSplitViewItem in Swift
问题 Almost all macOS official apps have this toolbar's feature which the NSToolbarItem flexible space is constraint to NSSplitViewItem view. I first thought maybe there are 3 different sections on NSToolbar. But it just ONE toolbar. You can open Notes app and customize the toolbar. The first flexible space is detected and moved along with first split view item. The second flexible space is detected and moved along with the second split item. The rest will be just normal flexible spaces. If there