In plasma-login-manager 6.5.91, the (very short) idle timeout never takes effect if there is a character in the password field (whether or not that field is active). You can leave it sitting for minutes at a time and it'll never go to the idle mode. Similarly, it never goes to idle mode if you've clicked 'Other' so the 'Username' field is visible - even if you don't actually type anything into Username or Password.