I added a lot of problems from the past Codeforces rounds to the mashup, now after adding the tasks I would like to sort them somehow by complexity or even manually drag and drop. Is there an option to do this somehow, or did you initially need to add tasks in the correct sequence?
I don't think there is an option for drag and drop. However, you can manually edit the problem IDs by clicking the icon next to the "Name" column title. The problems will then be displayed in sorted order by problem ID.