First of all I think that the Squeezer (issue #1529353) extension is
more useful and solves most of the problems that this proposed feature
is set to solve, e.g. issue #1442493.


IMO the second method you offer - temporarily moving "iomark" - is
preferable.

One reason is that it will allow using the undo feature to undo deleting
the entire shell history. I think this is important since deleting the
history by accident without a way to retrieve it would be very annoying.

I propose going with the clear_window2 method, wrapping it with
undo_block_start() and undo_block_stop().