Wrong viewport width with hidden columns in grading table

[troeger]

As reported by @werner-matthias:

Steps to reproduce:

• Use recent Safari on Mac Os X High Sierra to view grading table with many assignments
• Hide all columns
• Browser viewport width remains the same, although the table does fit into the window ("without scroll bars")

The effect does not show up on Firefox.

Might be related to the "autoWidth" setting in Datatables.