bug#38828: 26.3; Customized mode line breaks height of vertical scroll bar

[Amin Bandali]

Hi Eli,

[ Sorry if this isn’t the best place to report this ]

With 37f9182b68c62ee1912cf28d4ea0c30b4f8d59e0 (which cites this bug
report), I’ve noticed some weird display issues in a few different
modes:

- In `erc-mode' (the main ERC mode, used for for IRC channel buffers),
  if I press M-v to scroll to an earlier point in buffer and then scroll
  down line by line[1], the line previously at the bottom of the screen
  starts getting repeated, rather than the actual buffer content.  One
  way to get rid of this is to jump to the end of the buffer with M->.
  Also, I noticed that parts of the buffer that react to mouse hover
  (e.g. buttonized links) get “fixed” (the correct text appears) once
  the mouse cursor passes over them.

- In `magit-revision-mode' (displays the details of a commit), parts of
  the diff (IIRC the last line visible) has a different background than
  its surrounding lines.

In both of the above cases, the issue appears to be like some kind of
‘update’ not taking place anymore.  In both cases, the ‘artifacts’ are
gone if the frame is resized or toggled full-screen.  It’s quite likely
that the issue will happen with other modes as well, but the above two
are ones I’ve noticed and reliably reproduced so far.

Sorry if this isn’t too helpful of a report, as I’m not at all familiar
Emacs’s display code.  But I’ll be happy to try and help pin point the
issue if you could tell me how. :-)

Thanks,
amin

Footnotes:
[1]  I have the following scroll-related settings in my init file:

     (setq scroll-step 1
           scroll-conservatively 10
           scroll-preserve-screen-position 1)

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