Grader report's floating headers break when grade_report_showaverages is off

Create a test course of size S. Go to the grader report for the course. Observe that the floating headers generally work on scroll as you'd expect, with a floating footer for column averages. Scroll to the bottom of the report. Observe that as you scroll right, there's a header on the bottom left marked Overall average for the averages. Go to the grader report settings (Site administration -> Grades -> Report settings -> Grader report) and uncheck the box for Show column averages . Go back to the grader report for the course. Observe that the floating headers continue to work, albeit without the activity averages and the header for them. Go back to the grader report settings and check the box for Show ranges . Go back to the grader report for the course. Observe that the floating headers continue to work, again without the floating footer for averages. Scroll to the bottom of the report. Observe that as you scroll right, there's a header on the bottom left marked Range for the activity grading ranges. Re-enable column averages. Observe that the floating headers work on scroll as you'd expect, with a floating footer for column averages. Scroll to the bottom of the report. Observe that as you scroll right, there's a header on the bottom left marked Range for the activity grading ranges. Observe that as you scroll right, there's a header on the bottom left marked Overall average for the averages.