bug#32523: 27.0.50; Emacs hangs when killing rectangle

[Joseph Mingrone <jrm@ftfl.ca>]

On Fri, 2020-08-21 at 07:06, Stefan Kangas <stefan@marxist.se> wrote:

> Eli Zaretskii <eliz@gnu.org> writes:

>>> > The rectangle does eventually get cut for me as well.  Ignoring speed,
>>> > the problem is that Emacs is unusable afterwards.  For example, if I go
>>> > away for an hour or so, then return, the Emacs process will still be
>>> > using something close to 100% CPU and trying to doing something simple,
>>> > like moving the point forward, may take minutes.

>>> Are you still seeing this behaviour?

>> I bet he does.  the problem with the slow responses after cutting the
>> rectangle is that Emacs performs redisplay each type the user types
>> some character.  The redisplay can be very small and optimized, but it
>> can also be much more thorough; for example, typing "M-x" typically
>> triggers a thorough redisplay.  Each time we need to perform a
>> non-trivial redisplay, the same problem with long lines hits again.

> Ah, so you interpret what he writes to mean that he leaves Emacs _in the
> same buffer_ and then sees these results?  Yes, that makes sense.  I
> somehow assumed he meant that this was persistent even after closing the
> problematic buffer, but he didn't say that explicitly.

> Asking the same questions here as in another bug report:

> Is there anything more we can/should do in this case short of rewriting
> the display engine?  Does it make sense to track this limitation in a
> bug report?

> etc/PROBLEMS says:

> *** Editing files with very long lines is slow.

> For example, simply moving through a file that contains hundreds of
> thousands of characters per line is slow, and consumes a lot of CPU.
> This is a known limitation of Emacs with no solution at this time.

> Best regards,
> Stefan Kangas

Hello Stefan,

I just tested the recipe in GNU Emacs 28.0.50 (build 1,
amd64-portbld-freebsd12.1, GTK+ Version 3.24.20, cairo version 1.16.0)
and it's still an issue in that Emacs is difficult to use after
performing the rectangle kill.  However, after the operation completes
*and I kill the buffer visiting the large file*, Emacs become responsive
again.  It's been a long time, but I recall that this wasn't the case in
the past.  I recall having to kill and restart Emacs before it became
usable again.

Joe