Incorrect row count when collapsing is enabled in the Alignment Editor

Scenario:

1. Open "data/samples/CLUSTALW/COI.aln".
2. Enable the collapsing mode.
   Expected state: a collapsing group appears, there are 17 visible rows.
   Current state: a collapsing group appears, there are 17 visible rows, but a row position label on the status bar shows that there are 18 rows. If you select the last row, the label will show position 17/18. It should show either 17 total rows or the row position should be calculated taking into account not visible rows.