table.header that are on the side of the table

Description

Table headers are super useful, because they repeat when they have a page break. But sadly they do not work when the header is places on the side and not on the top of the table.

This example shows the issue quite nicely. The header fills an entire row. But what if I want just the left cell to be a header and fill the rest with other data. The cells next to the header are empty although they should be filled with the x cells.

#table(
  columns: 2,
  table.header(table.cell(rowspan: 3)[test]),
  [x],
  [x],
  [x],
  [x],
  [x],
  [x],
)

Use Case

This is use full in a case like this:

#table(
  columns: 2,
  table.cell(rowspan: 3)[100],
  [100a],
  [100b],
  [100c],
  table.cell(rowspan: 3)[200],
  [200a],
  [200b],
  [200c],
)

This looks like it should. Three numbers on the right are associated with one on the left.

But in case of a pagebreak it looks like this. The numbers on the next page can not be associated correctly. If a table header would allow it to be vertical and repeatable, it would solve this issue.

Start by reproducing the two table examples in the issue, especially the page-break behavior with a side table.header. Then trace how table.header handles row and column spans during pagination. Done means a vertical header repeats on subsequent pages while adjacent cells retain the expected contents and existing table behavior remains intact.