I did it making the height of the modal a fixed value:
#modalShare {
margin-left:-200px;
height: 300px;
}
@media (max-width: 767px) {
#modalShare {
width: auto;
margin-left: auto;
}
}
#modalShareBody {
overflow:hidden;
}
#panelGuest{
width: 35%;
float:left;
height: 200px;
overflow-y: auto;
overflow-x: hidden;
border-right:solid #ff920a 2px;
}
#panelDetail{
float:left;
width: 60%;
}
Is that what you need?