bug#29664: 26.0.90; Auto Hscroll Mode + Visual Line Mode + Truncate don't work well together

[Eli Zaretskii <eliz@gnu.org>]

> From: Kaushal Modi <kaushal.modi@gmail.com>
> Date: Mon, 11 Dec 2017 21:07:50 +0000
> Cc: 29664@debbugs.gnu.org
> 
>  Why are you turning on both visual-line-mode and truncate-lines?
>  These are mutually exclusive: the former is for wrapping long
>  continued lines, the latter is for not continuing lines at all.
> 
>  What am I missing?
> 
> Yes, I understand that. But they were both enabled by mistake. 
> 
> - The visual line mode was enabled by default in org-mode-hook
> - And I happened to have "# eval: (toggle-truncate-lines 1)" in the Local Variables footer in one of the Org files.
> 
> There were no signs of problem until C-a and C-e started misbehaving that way. 
> 
> Should emacs do the "right thing" and auto-disable the other if one got enabled, or throw a warning?

I'm not sure it should, but at least what you see is expected when you
mix these modes, they are incompatible and utterly confuse the display
engine.

> This problem with C-a and C-e does not happen if skip this part in the test setup:
> 
>  (setq auto-hscroll-mode 'current-line)
> 
> - C-a works perfectly
> - C-e does not jump instantly to the EOL, but progressively goes to the EOL on hitting C-e, and eventually gets
> there (interesting behavior).

(So not really "no problem".)

> So at least with the default value of auto-hscroll-mode, a user is not stuck wondering why C-a/C-e stopped
> working, even if they enabled visual-line-mode and toggle-truncate-lines simultaneously by mistake.

If someone wants to propose a patch to warn the user in this case, I
don't object.

> PS: I noticed that the email client or the debbugs server auto-inserted newlines in my test function.

I see no such newlines in the mail message I received.