The window is definitely a grid datawindow, and the test using dot notation is definitely working.
Yes, actually, the data is being copied in with RowsCopy(). I have moved the setting of the Width.Autosize properties to after the RowsCopy() (previously it was before it) and now both the values 2 and 3 work, to some extent.
However, only to some extent. One column that was too narrow without that setting is now wide enough for all of the data in it. But two other columns are still much wider than the data.