Once the Grader looks at the details of a submission, possibly consisting of several files, she/he can select one file and can begin to mark this particular files (adding comments, etc). The submitted file from the student-group/submission-group should not be modifiable.
Note: Maybe add link to move quickly to next submission assigned to the Grader.
* Switch between different files of a submission and test if changes or uncommitted modification in comments are preserved, or a warning pops-up.