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HTML:

<table>
  <thead>
    <tr>
      <th>A</th><th>B</th><th>C</th><th>D</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>E</td><td>F</td><td>G</td><td>H</td>
    </tr>
  </tbody>
</table>

Select any set of cells across the two rows and try merging them. They are merged correctly, but it looks as if the cells in the second row disappeared. This is because HTML 'rowspan' attributes can not cause cells to span multiple table sections.

We'd need to merge the sections when this happens, or just prevent the user from merging the cells.