the longer file on the left side,
enable Edit mode, put or keep Cursor in left panel.
- place cursor in left panel in the middle of the additional line on the end (i.e. line three of five)
- press Ctrl-right to the line copy to right panel
- press Ctrl-Z (or Alt-Backspace) twice to undo the inserting of the line
=> The inserted line is not deleted, it stays with line number 0.
=> The position of the inserted line is moved one line down.
=> Doing the same with the last line on the left will move the inserted line on the right lower than the last line on the left.
=> Every undone inserted new line moves the existing "new lines" one more line down.