The TableSizeComputer.compute() method can return cells with incorrect cell indexes. This can result in the _addColumn function only adding a column to some rows in the report editor.

TableSizeComputer.compute() does not handle cases where a conditional is used inside of a th element but the corresponding td element does not have a conditional.

Steps to reproduce
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1. Open the Delivery Slip in the report editor.
2. Click the header to show the dropdown and select elif: o.move_line_ids and o.state=='done'.
3. Hover over the last th element to select the column button.
4. Click Insert right and see that only the header row has an additional column.

Cause
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TableSizeComputer.compute() calculates how many possible cells can be in a row which can be more than the total number of columns you have if you have th or td elements with a conditional inside of them.

In the Delivery Slip report, a th element is present when using serial numbers and a different th element is present when not using serial numbers. The corresponding t-body has two different tr elements for these conditions. The compute method will have one less cell in these rows because the corresponding conditional is handled outside of the row.

Solution
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Only count mutually exclusive th or td elements once. This prevents the cellIndex from being larger than the total possible columns.

opw-ticket 5173345

Forward-Port-Of: odoo/enterprise#98207