Hi,
It is a CSS problem.
The popup display a joomla page in a "popup" context but your css defines a minimum width size for the page.
That's why there is a scrollbar in the popup.
If you want to test the "popup" context, just add the parameter "&tmpl=component" to an url of your website.
The template of your website won't be display, just his content (but CSS are still include in the page).
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.