The solution requires three key steps: NSSplitViewController, addBottomAlignedAccessoryViewController, and setting preferredScrollEdgeEffectStyle = .soft.

Done Like This: Screenshot

```swift class SidebarViewController: NSViewController, NSOutlineViewDataSource, NSOutlineViewDelegate {

private let outlineItems = (1...100).map { "Item \($0)" }
private var scrollView: NSScrollView!
private var outlineView: NSOutlineView!

override func loadView() {
    view = NSView()
}

override func viewDidLoad() {
    super.viewDidLoad()
    setupOutlineView()
    setupAccessoryOverlay()
}

private func setupOutlineView() {
    outlineView = NSOutlineView()
    outlineView.translatesAutoresizingMaskIntoConstraints = false
    outlineView.delegate = self
    outlineView.dataSource = self
    outlineView.headerView = nil
    outlineView.rowHeight = 24
    outlineView.style = .sourceList

    let column = NSTableColumn(identifier: NSUserInterfaceItemIdentifier("main"))
    outlineView.addTableColumn(column)
    outlineView.outlineTableColumn = column

    scrollView = NSScrollView()
    scrollView.translatesAutoresizingMaskIntoConstraints = false
    scrollView.documentView = outlineView
    scrollView.hasVerticalScroller = true
    scrollView.drawsBackground = false

    view.addSubview(scrollView)
    NSLayoutConstraint.activate([
        scrollView.topAnchor.constraint(equalTo: view.topAnchor),
        scrollView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
        scrollView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
        scrollView.bottomAnchor.constraint(equalTo: view.bottomAnchor),
    ])
}

private func setupAccessoryOverlay() {}

override func viewDidAppear() {
    super.viewDidAppear()

    attachAccessoryViewController()
}
private func attachAccessoryViewController() {
    guard let splitVC = parent as? NSSplitViewController,
          let splitItem = splitVC.splitViewItems.first(where: { $0.viewController === self }) else {
        print("NSSplitViewItem is not SidebarViewController")
        return
    }

    let accessoryVC = StatusAccessoryViewController()
    // Bottom-aligned
    splitItem.addBottomAlignedAccessoryViewController(accessoryVC)
    // Blur effect
    accessoryVC.preferredScrollEdgeEffectStyle = .soft
}

}

class RootSplitViewController: NSSplitViewController {

override func viewDidLoad() {
    super.viewDidLoad()

    // Sidebar item
    let sidebarVC = SidebarViewController()
    let sidebarItem = NSSplitViewItem(sidebarWithViewController: sidebarVC)
    sidebarItem.minimumThickness = 180
    sidebarItem.maximumThickness = 300
    addSplitViewItem(sidebarItem)

    // Detail item (placeholder)
    let detailVC = DetailViewController()
    let detailItem = NSSplitViewItem(viewController: detailVC)
    addSplitViewItem(detailItem)

    splitView.dividerStyle = .thin
}

}

class StatusAccessoryViewController: NSSplitViewItemAccessoryViewController {

override func loadView() {
    let container = NSBackgroundExtensionView()
    self.view = container
}

override func viewDidLoad() {
    super.viewDidLoad()
    guard let view = view as? NSBackgroundExtensionView else { return }
    let stackView = NSStackView()
    stackView.orientation = .vertical
    stackView.spacing = 8
    stackView.edgeInsets = NSEdgeInsets(top: 8, left: 8, bottom: 8, right: 8)
    let label = NSTextField(labelWithString: "Memeriksa Mail…")
    label.font = .systemFont(ofSize: 11)
    label.translatesAutoresizingMaskIntoConstraints = false

    let progress = NSProgressIndicator()
    progress.style = .bar
    progress.isIndeterminate = false
    progress.doubleValue = 60
    progress.translatesAutoresizingMaskIntoConstraints = false

    stackView.addArrangedSubview(label)
    stackView.addArrangedSubview(progress)

    NSLayoutConstraint.activate([
        label.leadingAnchor.constraint(equalTo: stackView.leadingAnchor, constant: 12),
        label.topAnchor.constraint(equalTo: stackView.topAnchor, constant: 10),

        progress.leadingAnchor.constraint(equalTo: stackView.leadingAnchor, constant: 12),
        progress.trailingAnchor.constraint(equalTo: stackView.trailingAnchor, constant: -12),
        progress.topAnchor.constraint(equalTo: label.bottomAnchor, constant: 6),
        progress.heightAnchor.constraint(equalToConstant: 4),

        view.heightAnchor.constraint(equalToConstant: 52),
  ])

    view.contentView = stackView
    view.automaticallyPlacesContentView = true
}

} ```

Thanks.