th-has-data-cells fails on empty tables

[just-boris]

I ran my pages using axe checker and it seems to fail on empty tables:

<table>
  <thead>
    <tr>
      <th>Product</th>
      <th>Amount</th>
      <th>Price</th>
    </tr>
  </tbody>
  <tbody>
    <tr>
      <td colspan="3">No available products to display</td>
    </tr>
  </thead>
</table>

Live demo https://jsbin.com/pewali

Expectation: axe handles this table and does not report incomplete checks

Actual: axe stumbles on this table and reports incomplete check

Motivation: Incomplete checks still require attention and should not give false-negatives

axe-core version: 3.5.5
chrome-extension: 4.5.3

[WilcoFiers]

Yup, you seem to have found a bug! Thank you for reporting. We'll definitely fix this.

[straker]

Note on the bug: It seems our getHeaders and getCellPosition functions do not take into account row/col-span. So when we look at the cells for the 2nd and 3rd headers and ask what the header of those cells are, it only returns the first header and not the other 2, so we fail saying that the cell points to a different header.

[padmavemulapati]

Verified with the latest axe-core(dated 22/10/2020) - 4.0.2 version
No incomplete results found for the table' rowspan and colspan

Also verified on 3.5.4 version where it is showing up incomplete results. (So fix is verified on 4.0.2 version)