Maybe you need to set the window style to NSWindowStyleMaskFullSizeContentView.

Yes, there are some borders. So just make the image you insert into icon composer a bit smaller, and that's it. The colors look the same here, but it could be your original image hadn't a color profile assigned, and it looked differently on a DCI-P3 display. Assign a color profile before importing it in Icon Composer.

Attach them after the CVPixelBufferCreate call. The attributes dictionary that can be passed to CVPixelBufferCreate is just to setup the pixel format, not to attach other attributes. So call CVBufferSetAttachments after the CVPixelBuffer is created and set them. P.S.: modifying CVPixelBuffer attributes is quite expensive, do it only as needed.

Is your app sandboxed? If so, the only way is to show a NSOpenPanel and ask the user to select the Documents folder (or any other folder). Showing an open panel and asking the user where to put the files is the best option even if the app is not sandboxed. Please don't litter the Documents folder with random things by default.

https://developer.apple.com/documentation/security/accessing-files-from-the-macos-app-sandbox

The destination folder should be configurable, not hard-coded to the Documents folder.

Like the message said, NSKeyedUnarchiver will require a complete list of all possible types for better security in the future.

Because NSArray can contain only objects, and NSNumber is the class that has been used to box bools, integers, floats, and double since a long time ago.