Scroll
on ul element
will not work, try to apply width-height
on outer div#Mainnavpanel-popup
like,
#Mainnavpanel-popup{
width: 300px;
height: 200px;
overflow: auto;
}
Updated if it should be visible then apply css on class ui-listview
like,
.ui-listview{
width: 300px;
height: 200px;
overflow: auto;
}