Incorrect column index calculation when cells are joined

[brunoagretti]

Describe the bug

When applying styles to header/footer cells, the column index is not calculated correctly if cells are joined.
This happens in HeaderFooterStylesHelper, method:

private int getColumnIndex(int rowIndex) {
  ROW row = getRowSelector().getRow();
  int j = -1;
  for (Column<?> c : helper.getGrid().getColumns()) {
    if (c.isVisible()) {
      ++j;
      if (getCell(row, c) == getCell()) {
        break;
      }
    }
  }
  return j;
}

The variable j keeps incrementing regardless of whether the cell has been joined, which results in wrong indexes being used and styles not being applied.

Expected behavior

The index calculation should consider joined cells so that styles are applied to the correct column.

Minimal reproducible example

No response

Add-on Version

1.4.0-SNAPSHOT

Vaadin Version

23.3.32

Additional information

No response

[brunoagretti]

Fix #135 fixed styles (cassnames) not being applied to all the cells in the header row, except for the second cell group, as seen here:

[javier-godoy]

As we discussed during our call, it seems that Grid reuses HeaderCell instances (the ColumnGroups, however, seem to remain constant)