A couple years back, I had some fun proof-of-concepting the terrible UX of preventing password managers or pasting passwords.

It can get so much worse than just an alert() when right-clicking.

The codepen.

A small note: It doesn't work with mobile virtual keyboards, since they don't send keystrokes. Maybe that's a bug, or maybe it's a security feature ;)

But yeah, best tried with a laptop or desktop computer.

How it detects password managers:

Unexpected CSS or DOM changes to the input element, such as an icon overlay for LastPass.
Paste event listening.
Right clicking.
Detecting if more than one character is inserted or deleted at a time.

In hindsight, it could be even worse by using Object.defineProperty to check if the value property is manipulated or if setAttribute is called with the value attribute.