bug#64855: 30.0.50; ERC 5.6: Make scrolltobottom less erratic

[J.P.]

Tags: patch

A minor bug fix involving the `scrolltobottom' module (and originally
lumped in with the mostly unrelated bug#63595) eventually materialized
on HEAD as [1]. While it did fix the bug, it didn't really address any
of the related foundational issues that have haunted `scrolltobottom'
since the beginning. The attached patch is meant to make the module more
predictable and consistent overall, while specifically addressing

  - the effect only being applied in the selected window

  - the effect not responding to changes in window size

  - the gradual scrolling of point toward the centermost line when
    away from the prompt (on graphical displays)

People miffed by this module's rough edges are encouraged to try these
changes and give feedback.

Thanks.


[1] commit e51e43b7046b56c58310854182a1d589ee4c770c
    Author: F. Jason Park <jp@neverwas.me>
    Date:   Wed May 17 19:48:02 2023 -0700

      Fix buffer-mismatch bug in erc-scroll-to-bottom

      * lisp/erc/erc-goodies.el (erc-scroll-to-bottom): Only
      `recenter' when the selected window's buffer is current.
      Previously, the module `scrolltobottom' signaled an "Error in
      `post-command-hook'" when a user clicked a channel indicator in
      the mode line from a window showing another ERC buffer.
    [...]
      (Bug#63595)

    lisp/erc/erc-goodies.el | 1 +
    lisp/erc/erc-track.el   | 9 ++++++++-
    2 files changed, 9 insertions(+), 1 deletion(-)


In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.38, cairo version 1.17.6) of 2023-07-25 built on localhost
Repository revision: 89558533683a100ca7946c4a35bf4ef50463efef
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12014000
System Description: Fedora Linux 37 (Workstation Edition)

Configured using:
 'configure --enable-check-lisp-object-type --enable-checking=yes,glyphs
 'CFLAGS=-O0 -g3'
 PKG_CONFIG_PATH=:/usr/lib64/pkgconfig:/usr/share/pkgconfig'

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JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NOTIFY
INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS WEBP X11 XDBE XIM XINPUT2 XPM GTK3 ZLIB

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Load-path shadows:
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Memory information:
((conses 16 64796 9162) (symbols 48 8637 0) (strings 32 23386 1972)
 (string-bytes 1 680274) (vectors 16 15042)
 (vector-slots 8 207499 8948) (floats 8 24 41) (intervals 56 225 0)
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0001-5.6-Consider-all-windows-in-erc-scrolltobottom-mode.patch
Description: Text Data