"npm i" displays broken output ("Enter passphrase for key...") and spinner, and entering passphrase does nothing

This is on Ubuntu 16.04. Terminal emulator is Konsole 15.12.3. Node.js is 4.8.2, installed as described here.

What happens:

$ npm i
npm WARN package.json my-proj@0.1.0 No repository field.
npm WARN package.json my-proj@0.1.0 No license field.
-nter passphrase for key '/home/stefan/.ssh/id_rsa': Enter passphrase for key '/home/stefan/.ssh/id_rsa': /

That's how it displays it, with the duplication, with the (non-spinning) / at the end and with the (spinning) - instead of the E in Enter.

If I just press enter at this point, I get errors such as Permission denied (publickey). - understandable, as I haven't entered my passphrase.

If I enter my passphrase instead (while the cursor is at the spinner, and nothing visibly changes while typing), and then press enter, I get one more row of:

-nter passphrase for key '/home/stefan/.ssh/id_rsa': \

... and nothing else happens. If I then press enter again, I get messages such as Permission denied (publickey). again.

via Stefan Monov