Fixed header with horizontally scrollable table

Hi everyone

I want to have a table where the header and the leftmost column is fixed and the columns can be scrolled horizontally. I tried this:

fixedColumns: true,
fixedHeader: true,
scrollX: true

The problem is when I scroll down so the header stays fixed at top, he expands. He behaves like expected (stays the same size as the table and is horizontally scrollable) when he's not at his fixed position but as soon as you scroll down he expands to his full size.

Is there a way to make this work?

Thanks for your time