Hi,
In DispForm it will open in a new form won’t it? If you set it readonly in EditForm / NewForm it will NOT open in a new window, but if you add this to your Custom JS in EditForm it should work:
jQuery(".ms-formbody").on("mouseover",".dffs-readonly-inner",function(){
jQuery(this).find("a").attr("target","_blank");
});
Let me know how this works out.
Alexander