[Table] Sticky header in group

When the stickyHeader option is enabled on a Table and the table contains multiple header rows, only the last of the rows ends up being sticky.

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

Only the last of the header rows is sticky.

Expected Behavior 🤔

All header rows should be sticky.

Steps to Reproduce 🕹

https://codesandbox.io/s/ymv2c

Tech	Version
Material-UI	v4.11.0

[oliviertassinari]

          <TableHead>
            <TableRow>
              <TableCell align="center" colSpan={columns.length}>
                Another header row
              </TableCell>
            </TableRow>
            <TableRow>
              <TableCell
                style={{ top: 57 }}
                align="center"
                colSpan={columns.length}
              >

https://codesandbox.io/s/material-demo-forked-g12em?file=/demo.tsx

[oliviertassinari]

I presume it’s not possible to apply the stickiness props to the element itself in order to avoid the custom top offset styling.

It’s not possible, to apply the sticky position on the thead, as far as I know. This would have been ideal.