So I used the password manager feature for the first time yesterday. I generated a new random password in the nitrokey app, and copied it to the clipboard. In a terminal window I pasted “it” for setting up full disk encryption (luks).
After copying terabytes of data, taking several hours, to that new encrypted drive, I disconnected it and wanted to unlock it on another computer. But I was unable to do so.
The reason is, I tried to paste the password via middle-click. Which pastes the X primary, not the X clipboard. And the nitrokey app only copied to the clipboard 
So I encrypted the drive with whatever random stuff I had selected at that time. Now I had to start from the beginning, wasting hours of time and terabytes of drive wear.
KeepassXC does it the right way, and copies to both clipboards.
Please change the app to fill both clipboards!