whole screen is obscured by many lines

Site changed to mobile version, but just after load many lines appears and the site can't be seen.
i tried install the add remove extension to see if work but still nothing.

It's facebook chat popup, add the following rule to templates/mobile_iphone/css/custom.css

#jfbcchat_base{display:none}

to hide it at all, or 

#jfbcchat_base{min-height:auto !important}

to display it correctly (but I'm not sure that it works correctly on mobile)