bug#24109: 24.5; Long lines in message mode make Emacs irresponsive

[Yuri D'Elia <wavexx@thregr.org>]

[-- Attachment #1: test.el.gz --]
[-- Type: application/gzip, Size: 2739 bytes --]

[-- Attachment #2: 1475346254.png --]
[-- Type: image/png, Size: 52667 bytes --]

[-- Attachment #3: Type: text/plain, Size: 1302 bytes --]

On Sat, Oct 01 2016, Yuri D'Elia <wavexx@thregr.org> wrote:
> I noticed one thing while trying to restrict the problem: setting
> auto-hscroll-mode to nil, an error is shown in the *Messages* buffer
> instead of entering an infinite loop:
>
> previous-line: Beginning of buffer
>
> while not moving the cursor at all on the first invocation, but working
> on the second. If something is off there, it might as well be trigger
> horizontal scrolling and thus conflict with the current goal column.

Ok, after some testing, I was able to get a self-contained test case.
I was successfully able to test this into the current git's master with
emacs -q, built with lucid as the main toolkit.

Please load-file the attached sample. It will create a *test* buffer,
using a quoted representation of (buffer-substring) which is sufficient
to recreate the problem.

It will move the point to the beginning of the last line, as well as
switching on truncate-lines. Having the point on the beginning of the
line is important.

Switch to the *test* buffer and move up. It will hang at the "Luciaine"
line. I'm also attaching a screenshot of my current window state.

If you toggle auto-hscroll-mode instead, before scrolling, you'll see
how the cursor disappears instead when exactly at the beginning of the
line.