Drop down Filter not shown with fixed header

Xoxxoxuatl

Link to test case: https://live.datatables.net/xehexoye/2102/edit

Hello, i am currently struggling with a z-index issue.
In the attached example the column 6 "Field 6" has a drop down as filter.
When the table data is scrolled (at a small window size), the drop down list is displayed behind the table data.

How can the drop down field also be shown when the table is scrolled?

Thanks and best regards,
Thomas