Fix incorrect upcast scan table heuristics responsible for creating heading rows.

[Mati365]

Suggested merge commit message (convention)

Fix (table): Prevent table corruption when setting editor data with th cells following colspan rows. Closes #17556, #17404

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Additional information

Our table upcasting does not recognize colspans correctly during calculation of the header rows. This PR ensures that rows that follow the rows with colspans are not longer used as table headings.

Check this HTML

	<figure class="table">
 	<table>
 		<tbody>
 			<tr>
 				<th>&nbsp;</th>
 				<td colspan="2" rowspan="2">&nbsp;</td>
 			</tr>
 			<tr>
 				<th>&nbsp;</th>
 			</tr>
 			<tr>
 				<th>&nbsp;</th>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 			</tr>
 		</tbody>
 	</table>
 </figure>
 <figure class="table">
 	<table>
 		<tbody>
 			<tr>
 				<th>&nbsp;</th>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 			</tr>
 			<tr>
 				<th>&nbsp;</th>
 				<td colspan="2" rowspan="2">&nbsp;</td>
 			</tr>
 			<tr>
 				<th>&nbsp;</th>
 			</tr>
 			<tr>
 				<th>&nbsp;</th>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 			</tr>
 		</tbody>
 	</table>
 </figure>
 <figure class="table">
 	<table>
 		<tbody>
 			<tr>
 				<th>&nbsp;</th>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 			</tr>
 			<tr>
 				<th>&nbsp;</th>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 			</tr>
 			<tr>
 				<th>&nbsp;</th>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 			</tr>
 			<tr>
 				<th>&nbsp;</th>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 			</tr>
 			<tr>
 				<th>&nbsp;</th>
 				<td colspan="2" rowspan="2">&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 			</tr>
 			<tr>
 				<th>&nbsp;</th>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 			</tr>
 			<tr>
 				<th>&nbsp;</th>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 			</tr>
 			<tr>
 				<th>&nbsp;</th>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 				<td>&nbsp;</td>
 			</tr>
 		</tbody>
 	</table>
 </figure>

Before

After

[pszczesniak]

The fix is working as expected, didn't found any side effects 👍

[Proudloaf]

Great news that this one has been fixed, nice work! Any rough idea on when it might get released?

[Witoso]

Mid-Jan or early February.

[Proudloaf]

Excellent, thanks!

[Mati365]

@Proudloaf While we'll release it in the stable next year, there is a way to check it on our nightly builds. Keep in mind, that it's unstable and should not be used on production.

[Proudloaf]

Fair enough, we're planning on updating to 44.1.0 very soon anyway, so once we have that stable we'll give it a go on your nightly. Thanks