Extending user1124809 's answer (snippet below) so that the tbody and thead fit the width of the table:

Set display: block; for the table Set position: sticky; top: 0; for the header row.

To make tbody and thead fit the width of the table:

table {
   display: block;
   overflow-y: auto;
}

thead {
  position: sticky;
}

thead, tbody {
display: table;
width: 100%;
}

Note - Apologies user1124809, I did not have enough rep to comment on the answer with this additional information so I had to reply with my own answer.