Finder copy bundle folders

No big crashes to report today. But some nasty behaviour in Finder.app.
When trying to copying files from and too a bundle, an infinite spinner pops up. Killing Finder.app removes the spinner, the files not copied. Trying again yielded a grey folder at the destination location.

I got to do this strange action because of the bad design of the Arduino IDE. The libraries of an Arduino project reside inside the Arduino.app. So if you want to add a library to your project, you need to change your IDE. I could continue to rant about the bad software design of Arduino, but that's not the subject of this blog.
Before adding my custom libraries, and hacking some code there, I've made a backup of the libraries folder in Arduino.app. Now I wanted to recover a library from the backup, so I copied from and to the Arduino.app bundle, the destination location already existed. Finder can't handle it.

I went inside the Arduino.app with the Terminal, found that the folder was there, but empty. By using the Terminal, it's easy to just

  rm dir; cp ../libraries_backup/dir . 

So that's what I did, and what Finder couldn't do today.