What you need to do is to invoke dpc1.enableScrolling(); after the modal Panel is made visible.
- I'm not sure whether it's possible to hook the show event of the ModalPopupExtender on the client side (that would be the right place to put it).
- You could also try to invoke it manually and add the enableScrolling() call right after the show() call:
$find('mdlPopupExtender').show();
dpc1.enableScrolling();