Subject: bug#50234: 28.0.50; Horizontal scrolling doesn't keep the point in view

From: Eli Zaretskii <[email-masked]>
Date: Sat, 28 Aug 2021 11:35:32 +0300

tags 50234 wishlist
thanks

> From: Dima Kogan <[email-masked]>
> Date: Sat, 28 Aug 2021 00:40:57 -0700
> 
> Hi. This is an old bug; I've been observing it for years.

It isn't a bug, it might be a missing feature.  Horizontal scroll
commands were never coded to support scroll-preserve-screen-position.
Only the vertical scroll commands support it.

The documentation of scroll-preserve-screen-position says:

  Scroll commands should have the ‘scroll-command’ property
  on their symbols to be controlled by this variable.

But:

  (get 'scroll-left 'scroll-command) => nil

> 1. emacs -Q --eval '(global-set-key [C-M-right]  (lambda () (interactive) (let ((scroll-preserve-screen-position t)) (scroll-left 3))))'
> 
>    Emacs comes up with the default text in the *scratch* buffer
> 
> 2. (goto-char (point-min))
> 
>    Move the point to the beginning of the buffer. (point) evaluates to 1
> 
> 3. C-M-right
> 
>    We scroll to the right. The point was on the left edge of the screen
>    at position 1, which is now out of view. Emacs still draws the point
>    at the left edge of the screen, implying that the point was moved to
>    stay in-bounds. But this is not where the point actually is: (point)
>    still evaluates to 1. I expect (point) to be updated with the new
>    position

This is the expected behavior.  The Emacs manual says:

     If the text is scrolled to the left, and point moves off the left
  edge of the window, the cursor will freeze at the left edge of the
  window, until point moves back to the displayed portion of the text.

> 4. C-a
> 
>    Now, some commands behave strangely. For instance C-a should move to
>    the start of the line. This is now off-screen, so I would expect
>    emacs to scroll back so that we can see the beginning of the line.
>    But emacs does nothing: the point was at position 1, and it was moved
>    to position 1, so it doesn't see the need to scroll anything.

This is also expected, since horizontal scroll command affect the
automatic hscrolling, as described in the manual:

     If you use those commands to scroll a window horizontally, that sets
  a lower bound for automatic horizontal scrolling.  Automatic scrolling
  will continue to scroll the window, but never farther to the right than
  the amount you previously set by ‘scroll-left’.

Bottom line: if we want scroll-preserve-screen-position to affect
horizontal scrolling in some (as yet to be defined) way, we need to
code that.  Other than that, what you report here is the expected and
documented behavior.

Thanks.