Narrowing

Turn on Narrowing.
Ensure that there are non-editable line(s) at the bottom.

Issue 1:
Use “Delete Spaces at End of Lines” command.
** The command handles the whole document nevertheless.
** Unable to undo this step unless Narrowing is off.

Issue 2:
Select some editable lines.
Use “Join Lines” command.
Undo.
** The editable area is not reverted. The number of the editable lines decreased.

Issue 3:
** Unable to use “Delete Line(s)” command on the last editable line(s). (Use the command more than once and you can see the selection extend oddly.)
** Any way to delete the last editable line(s) leaves an empty line.
** Triple-clicking in an editable line selects the line including the newline character, except in the last editable line.