How can I robustly make (unknown) tables span the full page width?

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Thanks for all your help, I’ve converged now on this #show table solution that seems to work pretty well. The vertical height might not always be minimal but that’s not important, at least no input I’ve tested so far looks unreasonable.

// an empty table cell that stores its own layout space as metadata
#let measurement-cell(table-id, x) = table.cell(
  inset: 0pt,
  stroke: none,
  x: x, // set x explicitly to be robust against dangling cells in the table definition
  layout(s => [#metadata(s)#label(table-id)]),
)

#let measurement-cells(table-id, n) = range(n).map(i => measurement-cell(table-id, i))

#let full-width-table(tbl) = {
  let f = tbl.fields()
  let columns = f.remove("columns")
  let children = f.remove("children")

  context {
    // use a counter to generate the unique ID needed to store the metadata
    let table-id = counter("table-id")
    table-id.step()
    let table-id = "table" + str(table-id.get().first())
    
    // retrieve the resolved cell sizes from the first layout
    // iteration and give each column a proportional fr size,
    // this will expand all columns such that the table now has
    // full page width
    let computed-sizes = query(label(table-id))
    let columns-sizes = columns
    if computed-sizes.len() != 0 {
      columns-sizes = computed-sizes.map(md => calc.round(md.value.width.pt()) * 1fr)
    }
    table(
      columns: columns-sizes,
      ..f,
      ..children,
      // insert the invisible measurement cells in the last row
      ..measurement-cells(table-id, columns.len()),
    )
  }
}

#show table: it => {
  // this is a bit hacky, determine if the table was already modified
  // by a zero inset measurement cell that was appended, the assumption
  // being that normal cells wouldn't have zero inset
  let is-processed = it.children.last().at("inset", default: none) == 0pt
  if is-processed {
    it
  } else {
    full-width-table(it)
  }
}
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