contrib pages trigger "unsaved changes" warning automatically

contribution pages (and likely event registrations) are triggering the "unsaved changes" warning even when no changes have been made. you can easily test with the demo site. visit: http://d46.demo.civicrm.org/civicrm/contribute/transact?reset=1&id=2 – then either reload the page or try to navigate away.

the issue is caused by the billingcheckbox field, which is present if the top profile has address fields and the form has payment processing enabled. the initial value for that field is false. I think that as the page loads it's determined if sufficient fields are present to be able to use that option, in which case it is defaulted to true. that change triggers the unsaved warning.