bug#7041: comint seems to truncate from places other than the top

[Reuben Thomas via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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On Tue, 8 Dec 2020 at 16:43, Lars Ingebrigtsen <larsi@gnus.org> wrote:

> Reuben Thomas <rrt@sc3d.org> writes:
>
> > If I add comint-truncate-buffer to comint-output-filter-functions, and
> > set comint-buffer-maximum-size to 1024, then when a term buffer
> > reaches about that size, and I run a command that produces a lot of
> > output, then scroll back, some of the output is missing. In other
> > words, rather than truncate from the top of the buffer, some of the
> > truncation at least seems to happen much nearer the bottom.
>
> (This bug report unfortunately got no response at the time.)
>
> Do you have a step-by-step recipe to reproduce this error, starting from
> "emacs -Q"?
>

If I try it now, it doesn't seem to truncate the buffer as expected: for
example

emacs -Q
M-: (add-to-list 'comint-output-filter-functions 'comint-truncate-buffer)
M-x term
C-c M-x set-variable comint-buffer-maximum-size 256 ; do this after M-x
term because before comint-buffer-maximum-size is not defined
$ seq 1 1024

The entire output is retained, and not truncated. Truncation seems to
happen at 2048 lines. This does not even correspond to the original value
of the variable (1024).

However, I cannot detect any truncation at the bottom of the output.

If I run C-c M-x comint-truncate-buffer, the buffer is truncated as
expected, to 256 lines.

-- 
https://rrt.sc3d.org

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