Auto-fill dropdown appears top left on customer account checkout step

[wiffbi]

Describe the bug
When using customer accounts, the login-form on checkout step is hidden behind a radio-box for „Log in with a customer account“. The form elements themselves are not disabled, which causes an auto-fill dropdown to appear top left on the page – at least in Firefox with having two customer accounts saved.
One could argue this is a browser issue, but already a long reported one. See https://bugzilla.mozilla.org/show_bug.cgi?id=1247245

Expected behavior
Have the form inputs disabled when hidden, so (hopefully) auto-fill is not shown.