I'm curious if their are instances where a user might change form data without first focusing on the form element. For instance, can it be counted on that an input field will be focused (onFocus) prior to its value being changed?

I know that it can't be counted on to be secure, but for general use by everyday users, can one assume an element's onFocus event will be triggered prior to the user changing form data? If not, in what scenarios isn't this the case?