The fixed column and header feature when scrolling in the gradebook breaks under the following conditions:

1) Report settings > Grader Report > Enable AJAX (ticked)
http://[moodleroot]/admin/settings.php?section=gradereportgrader

2) Create course (using clean theme to prove core moodle bug)

3) Enrol 2 or more students

4) Add a couple of assignments with long titles (for the purposes of checking scrolling)

5) Open grader report and turn editing on

• Test the scrolling in the gradebook at this point - it should work

6) Lock a cell for one of the students (so the field disappears).

7) Reload the page after locking the field.

• Test the scrolling in the gradebook - it should no longer work