I'm using this solution https://stackoverflow.com/a/4932770/3108268
$("#EmailAddress").keyup(function(){
$("#Username").val(this.value);
});
And it works fine when typing input values with a keyboard, it auto fills the selected field.
But it doesn't work when filling a field with a mouse click from calendar popup javascript plugin. The popup plugin gives the input classes hasDatepicker date-popup-init.
This is all inside Drupal 7, but I don't think it's relevant.
Here's an example, as you can see the time field is plain text and autofills below, because it is being filled with a keyboard while filling the date field with mouse click from calendar popup does not fill the field below it (but does if you force-type with keyboard).
