MacBook Pro login screen stuck keeps pressing a key into password field forever

I found a solution for me inspired from https://discussions.apple.com/thread/3450678

A comment there mentioned bluetooth devices. There can be some issue with bluetooth and other devices that might not be actively used, especially if they are far away or low battery.

I turned off my bluetooth and it went away.

I had EXACTLY this problem - and the solution made me feel a bit stupid in the end. I had a bluetooth keyboard stored in a bag a few meters away, which must have had a key pressed.

Funny thing is it only repeated on the login screen?

But pulling it out the bag (or turning off bluetooth) sorted it straight away.

I found this question from my phone while experiencing the same problem (High Sierra). I have no bluetooth keyboard, just the laptop by itself and I had tried pressing all the keys individually (ruling out physical stickiness).

I was able to solve the problem by selecting "switch user". After selecting myself from the list of users, it switched me back to the login screen and the repeating key abruptly stopped (it entered 2 characters in the password field and that was it).

No shutdown was required. I'm not sure if this is an expected solution to the problem or if I just got lucky. Also, I'm not sure whether this workaround was enabled by having multiple accounts on the machine (such that "switch user" is a valid action in the first place). I'm also not sure if (as a distant possibility) the fix was related to my having to move and click the trackpad a bit.

But it worked, and hopefully it will work for you too.