Innocent users click entire columns before clicking Data>AutoFilter. The result is inacceptable.
Yes, LibreOffice in this case includes all rows of the sheet in the filter (and this is not reasonable in my opinion).
Excel in this case (as far as I know) will limit the filter area to the row for the last “used” cell of the selected columns. “Used” cell: non-empty cell, cell with direct formatting, …
Yet, it isn’t a problem with previous versions, and it’s expected for users to select whole columns for filtering. I’ve up to 700k rows in some sheets and it’s fast opening the filter window.
Just bizarre that “Clear Contents” works for 700001:1048576