Hi,
This problem is a style (CSS) problem with some joomla 3 templates.
I already fixed it for a customer but I have some difficulties to find the forum thread.
The edit button have the "modal" class even if it's joomla 3 (we use the bootstrap popup otherwise).
You should add a specific CSS rule for " span.hikashop_checkout_billing_address_buttons a.modal " in order to remove some "position absolute" or other specific "modal" rules.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.