Open Radar

 
1. Share a folder from a secondary Apple ID to you primary Apple ID
2. Open TextEdit on Mac signed into the primary Apple ID
3. Keep Finder window nearby to see the file sync progress of the file
4. Create a plain text file with some text at the root of the shared iCloud Drive folder
5. Open the same document in TextMate (optional, but useful for another confirmation of what’s happening)
6. Type some more text into the file. 
7. ⌘-tab to TextMate. Switching to another app causes TextEdit to autosave the file
8. Watch file sync progress until it settles down. If you didn’t see the file revert, repeat steps 6–8 until it does

You can see a screen recording of the issue at https://twitter.com/antons/status/1532444431793766419.

In the screen recording the file lost all recent modifications for 5 full seconds (5 bytes started → 50 bytes saved → 5 bytes reverted by iCloud Drive → 50 bytes), as shown by both TextEdit and TextMate.

This is consistently, repeatedly, easily reproduble when using TextEdit, iA Writer — both NSDocument and NSFilePresenter based apps. I couldn’t reproduce the issue when I saved the file only in TextMate, which does not use NSDocument and NSFilePresenter. 

When debugging this, you can breakpoint at -[NSDocument relinquishPresentedDataToWriter:] to catch the exact moment when iCloud Drive reverts the file.