Неверный интервал между элементами NSOutlineView

У меня есть NSOutlineView, и расстояние между элементами 1 и 2 больше, чем расстояние между всеми другими элементами. Это групповые предметы. Общий вид очень голый. Ниже мой код, а ниже - изображение проблемы, о которой я говорю:

var sections: [SidebarSection] = [
    SidebarSection(title: "Section 1", items: ["1", "2", "3", "4", "5"]),
    SidebarSection(title: "Section 2", items: ["1", "2"]),
    SidebarSection(title: "Section 3", items: ["1", "2"]), 
    SidebarSection(title: "Section 4", items: ["1", "2"]), 
    SidebarSection(title: "Section 5", items: ["1", "2"])]

private let outlineView: SidebarOutlineView = SidebarOutlineView()

extension SidebarViewController: NSOutlineViewDataSource {

    func outlineView(_ outlineView: NSOutlineView, isItemExpandable item: Any) -> Bool {
        return item is SidebarSection
    }
    
    func outlineView(_ outlineView: NSOutlineView, child index: Int, ofItem item: Any?) -> Any {
        if let section = item as? SidebarSection {
            return section.items[index]
        }
    
        return sections[index]
    }

    func outlineView(_ outlineView: NSOutlineView, numberOfChildrenOfItem item: Any?) -> Int {
        if let section = item as? SidebarSection {
            return section.items.count
        }
        return sections.count
    }
}

extension SidebarViewController: NSOutlineViewDelegate {

    func outlineView(_ outlineView: NSOutlineView, heightOfRowByItem item: Any) -> CGFloat {
        return 30
    }

    func outlineView(_ outlineView: NSOutlineView, isGroupItem item: Any) -> Bool {
        return item is SidebarSection
    }

    func outlineView(_ outlineView: NSOutlineView, viewFor tableColumn: NSTableColumn?, item: Any) -> NSView? {
        let view = outlineView.makeView(withIdentifier: NSUserInterfaceItemIdentifier(rawValue: "SidebarPlaylistCell"), owner: self) as! SidebarPlaylistCell
        view.wantsLayer = true
        view.textLabel.stringValue = (item as? SidebarSection)?.title ?? ""
        view.imgView.wantsLayer = true
        view.imgView.layer?.backgroundColor = NSColor.red.cgColor
        return view
    }
}

extension SidebarViewController {

   func layout() {
        scrollView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(scrollView)
    
        scrollView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
        scrollView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
        scrollView.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true
        scrollView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
    
        scrollView.documentView = outlineView
    
        outlineView.headerView = nil
    
        outlineView.delegate = self
        outlineView.dataSource = self
         outlineView.intercellSpacing = NSSize(width: 0, height: 0)
        scrollView.hasVerticalScroller = true
        scrollView.hasHorizontalScroller = false
        scrollView.wantsLayer = true
    
        view.layer?.backgroundColor = NSColor.blue.cgColor
    }
}


final class SidebarOutlineView: NSOutlineView {

    override init(frame frameRect: NSRect) {
        super.init(frame: frameRect)
        let column: NSTableColumn = NSTableColumn(identifier: NSUserInterfaceItemIdentifier("col1"))
        column.isEditable = false
        column.minWidth = 0
        addTableColumn(column)
        outlineTableColumn = column
    
        register(NSNib(nibNamed: "SidebarPlaylistCell", bundle: nil), forIdentifier: NSUserInterfaceItemIdentifier(rawValue: "SidebarPlaylistCell"))
        selectionHighlightStyle = .sourceList
        indentationPerLevel = 6.0
    }

    required init?(coder: NSCoder) {
        super.init(coder: coder)
    }
}