NSWindow Transparent Background NSCollectionView background not repainting

I am trying to create a macOS application that is semi-transparent. Like the Terminal application that is included with macOS. I created an NSWindow subclass that is transparent and a pretty standard NSCollectionView in a Storyboard. I can get the application to be semitransparent but when I scroll the NSCollectionViewItems it leaves the background fully transparent.

Code for the problem is here on Github

Example of Problem

I have tried many things

• Get the callbacks of the NSScroll View and get the UI to redraw
• Set the background in most of the views in the Storyboard
• The StackOverflow question's suggestions: StackOverflow

Here is the custom UIWindow

import Cocoa

class MainWindow: NSWindow {

    override init(contentRect: NSRect, styleMask style: NSWindow.StyleMask, backing backingStoreType: NSWindow.BackingStoreType, defer flag: Bool) {
        super.init(contentRect: contentRect, styleMask: style, backing: backingStoreType, defer: false)
        self.backingType = NSWindow.BackingStoreType.buffered
        self.alphaValue = 1.0
        self.backgroundColor = NSColor.clear
        self.isOpaque = false
    }
}

Here is the ViewController

import Cocoa

class ViewController: NSViewController {

    @IBOutlet weak var collectionView: NSCollectionView!
    @IBOutlet weak var scrollView: NSScrollView!

    override func viewDidLoad() {
        super.viewDidLoad()

        configureCollectionView()
        collectionView.register(ReceiverCollectionViewItem.self, forItemWithIdentifier: NSUserInterfaceItemIdentifier(rawValue: "Cell"))
    }

    private func configureCollectionView() {
        let flowLayout = NSCollectionViewFlowLayout()
        flowLayout.itemSize = NSSize(width: 100.0, height: 100.0)
        flowLayout.sectionInset = NSEdgeInsets(top: 10.0, left: 20.0, bottom: 10.0, right: 20.0)
        flowLayout.minimumInteritemSpacing = 20.0
        flowLayout.minimumLineSpacing = 20.0
        collectionView.collectionViewLayout = flowLayout

        scrollView.drawsBackground = false
        scrollView.contentView.drawsBackground = false
        scrollView.backgroundColor = NSColor(calibratedRed: 0.0, green: 0.0, blue: 0.0, alpha: 0.1)

        collectionView.backgroundColors = [NSColor(calibratedRed: 0.0, green: 0.0, blue: 0.0, alpha: 0.1)]
    }
}

extension ViewController: NSCollectionViewDataSource {
    func collectionView(_ collectionView: NSCollectionView, numberOfItemsInSection section: Int) -> Int {
        return 10
    }

    func collectionView(_ collectionView: NSCollectionView, itemForRepresentedObjectAt indexPath: IndexPath) -> NSCollectionViewItem {
        let item = collectionView.makeItem(withIdentifier: NSUserInterfaceItemIdentifier(rawValue: "Cell"), for: indexPath) 
        return item
    }
}

After much trial and error (more error) I found the solution.

In the NSWindow subclass if you set the background color this gets rid of the clear "holes" in the background.

import Cocoa

class MainWindow: NSWindow {

    override init(contentRect: NSRect, styleMask style: NSWindow.StyleMask, backing backingStoreType: NSWindow.BackingStoreType, defer flag: Bool) {
        super.init(contentRect: contentRect, styleMask: style, backing: backingStoreType, defer: false)
        self.backingType = NSWindow.BackingStoreType.buffered
        self.alphaValue = 1.0
        self.backgroundColor = NSColor(calibratedRed: 0.0, green: 0.0, blue: 0.0, alpha: 0.2)
        self.isOpaque = false
    }
}

The ViewController also required a few things

import Cocoa

class ViewController: NSViewController {

    @IBOutlet weak var collectionView: NSCollectionView!
    @IBOutlet weak var scrollView: NSScrollView!

    override func viewDidLoad() {
        super.viewDidLoad()

        configureCollectionView()
        collectionView.register(ReceiverCollectionViewItem.self, forItemWithIdentifier: NSUserInterfaceItemIdentifier(rawValue: "Cell"))
    }

    private func configureCollectionView() {
        let flowLayout = NSCollectionViewFlowLayout()
        flowLayout.itemSize = NSSize(width: 100.0, height: 100.0)
        flowLayout.sectionInset = NSEdgeInsets(top: 10.0, left: 20.0, bottom: 10.0, right: 20.0)
        flowLayout.minimumInteritemSpacing = 20.0
        flowLayout.minimumLineSpacing = 20.0
        collectionView.collectionViewLayout = flowLayout

        scrollView.drawsBackground = false
        scrollView.contentView.drawsBackground = false
        scrollView.backgroundColor = NSColor.clear

        collectionView.backgroundColors = [.clear]
    }
}

extension ViewController: NSCollectionViewDataSource {
    func collectionView(_ collectionView: NSCollectionView, numberOfItemsInSection section: Int) -> Int {
        return 10
    }

    func collectionView(_ collectionView: NSCollectionView, itemForRepresentedObjectAt indexPath: IndexPath) -> NSCollectionViewItem {
        let item = collectionView.makeItem(withIdentifier: NSUserInterfaceItemIdentifier(rawValue: "Cell"), for: indexPath) 
        return item
    }
}