[mahathu]

In the file settings ("Get Info") you can set a "stationary pad" flag means whenever the file gets edited, a copy is created, and the original file doesn't change.

When you're taking a screenshot of a portion of the screen you can hold the spacebar to move the rectangle around.

[xyzzy_plugh]

Is this Copy-on-Write? Is the copy a real file or is this a filesystem implementation detail?

[leejoramo]

The stationary feature has existed since the “Classic” days. Probably MacOS 7. So it comes from the HFS era.

It would be more accurate to say when you open a file with the Stationary flag set, a new of copy of the document is open in the default application. I don’t think a file system copy is created until you save.

[johndoe0815]

Stationary pads were already implemented in the Lisa UI, but it took quite some time to show up in classic MacOS - https://www.applefritter.com/node/3198

[OskarS]

APFS has CoW semantics, so I would assume it uses the APFS filesystem features.

[yjftsjthsd-h]

I'm pretty sure that feature is older than OS X, let alone APFS. It might be implemented using CoW now, but it certainly hasn't always been.