Looks like fixed headers for scrollable tables has been raised (and "resolved") before, but practically speaking it's never really been properly fixed.

There is CONFCLOUD-9267, that claims that things have been fixed long time ago, but practically that is not the case, as we can see here and here.

In practice, I should not care whether a table has:

• been embedded into page layouts/sections
• row numbers
• merged cells
• a horizontal scroll

The header should "just work"™.