https://stackoverflow.com/questions/63094016/big-sur-outline-view-expandable-items-broken

ViewController2.swift

// Finder like two levels

import Cocoa

class ViewController: NSViewController {
    @IBOutlet var outlineView: NSOutlineView?

    let data = [
        Node(1, "Group A", true, [
            Node(11, "A 1"),
            Node(12, "A 2"),
            Node(13, "A 3"),
            ]),
        Node(2, "Group B", true, [
            Node(21, "B 1")
        ])
    ]
}

extension ViewController: NSOutlineViewDataSource {
    override func awakeFromNib() {
        super.awakeFromNib()
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        data.forEach {
            outlineView?.expandItem($0)
        }
    }
    
    func outlineView(_ outlineView: NSOutlineView, shouldSelectItem item: Any) -> Bool {
        !(item as! Node).isGroup
    }
        
    func outlineView(_ outlineView: NSOutlineView, child index: Int, ofItem item: Any?) -> Any {
        item == nil ? data[index] : (item as! Node).children[index]
    }
    
    func outlineView(_ outlineView: NSOutlineView, isGroupItem item: Any) -> Bool {
        (item as! Node).isGroup
    }
    
    func outlineView(_ outlineView: NSOutlineView, isItemExpandable item: Any) -> Bool {
        (item as! Node).children.count > 0
    }
    
    func outlineView(_ outlineView: NSOutlineView, numberOfChildrenOfItem item: Any?) -> Int {
        item == nil ? data.count : (item as! Node).children.count
    }
    
    func outlineView(_ outlineView: NSOutlineView, objectValueFor tableColumn: NSTableColumn?, byItem item: Any?) -> Any? {
        item
    }
}


extension ViewController: NSOutlineViewDelegate {
    func outlineView(_ outlineView: NSOutlineView, viewFor tableColumn: NSTableColumn?, item: Any) -> NSView? {
        guard let node = item as? Node else {
            preconditionFailure("Invalid data item \(item)")
        }
        
        let identifier = NSUserInterfaceItemIdentifier(rawValue: node.isGroup ? "HeaderCell" : "DataCell")
        
        let view = outlineView.makeView(withIdentifier: identifier, owner: self) as? NSTableCellView
        view?.textField?.stringValue = node.name
        if !node.isGroup {
            view?.imageView?.image = NSImage(systemSymbolName: "folder", accessibilityDescription: nil)
        }
        return view
    }
}


final class Node {
    let id: Int
    let name: String
    let isGroup: Bool
    let children: [Node]
    
    init(_ id: Int, _ name: String, _ group: Bool = false, _ children: [Node] = []) {
        self.id = id
        self.name = name
        self.isGroup = group
        self.children = children
    }
}

ViewController3.swift

// Three levels

import Cocoa

class ViewController: NSViewController {
    @IBOutlet var outlineView: NSOutlineView?

    let data = [
        Node(1, "Group A", true, [
            Node(11, "A 1"),
            Node(12, "A 2"),
            Node(13, "A 3"),
            ]),
        Node(2, "Group B", true, [
            Node(21, "B 1", false, [
                Node(211, "B 1 1"),
                Node(212, "B 1 2")
            ])
        ])
    ]
}

extension ViewController: NSOutlineViewDataSource {
    override func awakeFromNib() {
        super.awakeFromNib()
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        data.forEach {
            outlineView?.expandItem($0)
        }
    }
    
    func outlineView(_ outlineView: NSOutlineView, shouldSelectItem item: Any) -> Bool {
        !(item as! Node).isGroup
    }
        
    func outlineView(_ outlineView: NSOutlineView, child index: Int, ofItem item: Any?) -> Any {
        item == nil ? data[index] : (item as! Node).children[index]
    }
    
    func outlineView(_ outlineView: NSOutlineView, isGroupItem item: Any) -> Bool {
        (item as! Node).isGroup
    }
    
    func outlineView(_ outlineView: NSOutlineView, isItemExpandable item: Any) -> Bool {
        (item as! Node).children.count > 0
    }
    
    func outlineView(_ outlineView: NSOutlineView, numberOfChildrenOfItem item: Any?) -> Int {
        item == nil ? data.count : (item as! Node).children.count
    }
    
    func outlineView(_ outlineView: NSOutlineView, objectValueFor tableColumn: NSTableColumn?, byItem item: Any?) -> Any? {
        item
    }
}


extension ViewController: NSOutlineViewDelegate {
    func outlineView(_ outlineView: NSOutlineView, viewFor tableColumn: NSTableColumn?, item: Any) -> NSView? {
        guard let node = item as? Node else {
            preconditionFailure("Invalid data item \(item)")
        }
        
        let identifier = NSUserInterfaceItemIdentifier(rawValue: node.isGroup ? "HeaderCell" : "DataCell")
        
        let view = outlineView.makeView(withIdentifier: identifier, owner: self) as? NSTableCellView
        view?.textField?.stringValue = node.name
        if !node.isGroup {
            view?.imageView?.image = NSImage(systemSymbolName: "folder", accessibilityDescription: nil)
        }
        return view
    }
}


final class Node {
    let id: Int
    let name: String
    let isGroup: Bool
    let children: [Node]
    
    init(_ id: Int, _ name: String, _ group: Bool = false, _ children: [Node] = []) {
        self.id = id
        self.name = name
        self.isGroup = group
        self.children = children
    }
}