bug#46316: 27.1; wrong horizontal scroll with truncate-lines value t

[ynyaaa@gmail.com]

Eli Zaretskii <eliz@gnu.org> writes:

>> Cc: 46316@debbugs.gnu.org
>> From: ynyaaa@gmail.com
>> Date: Mon, 08 Feb 2021 03:54:32 +0900
>> 
>> I only want auto-hscroll-mode to scroll buffer automatically.
>
> It does, when you give it enough information that it should.  When
> your input focus is in another window (the mini-window in the case of
> C-s), I think it would be strange for Emacs to auto-scroll some other
> window.
>
>> If a searched string is not displayed in the window, I might think the
>> searched string does not exist in the buffer.
>
> The searched string _is_ displayed, when you first find it.
>
>> If an image is hidden, I might think the image is a solid color same as
>> the emacs background color.
>
> Just move the cursor and you will see it.
>
> Sorry, I see no problem here, certainly not something that would
> justify complicated triggers for redisplaying a window.

I investigated some more.

#'isearch-update tries to control the hsrcoll using
#'pos-visible-in-window-group-p.
Under the folloing conditions, it returns coordinates as if the point is
at the beginning of a virtual next line.
  (1) the point is the end of the buffer.
  (2) the buffer does not end with a newline.
  (3) the last line is displayed in the window.
  (4) the last line is truncated.
So the calculation is wrong and the hscroll becomes inappropriate.

With a different reason, if isearch started with a large hscroll and the
last match is near the beginning of a line, the hscroll becomes
inapropriate.
For example, evaluate the form below, and type 'C-s a C-s', then the
current point is hidden to the left of the window.
This is because #'isearch-update only checks whether the point is to the
right of the window and does not check whether the point is to the left
of the window.

(let ((buf (generate-new-buffer "tmp")))
  (switch-to-buffer buf)
  (setq truncate-lines t)
  (insert-char ?x 100)
  (save-excursion
    (insert-char ?\n 100)
    (insert ?a)))

Regarding image-mode, a simpler example of the bug is written as below.
The hscroll does not change when the buffer is changed.
Image display, window focus or message is not related.

(let ((buf (generate-new-buffer "tmp")))
  (switch-to-buffer buf)
  (setq truncate-lines t)
  (dotimes (i 200)
    (insert (format ":%03d" i))
    (when (= 0 (% (1+ i) 100)) (insert ?\n)))
  (forward-char -1)
  (sit-for 1)
  (set-window-hscroll nil (window-hscroll))
  (save-excursion
    (forward-char -100)
    (insert-char ?\n 10)
    ))