bug#26994: 26.0.50; display bugs with auto-hscolling current line

[Stephen Berman]

On Sat, 20 May 2017 10:37:32 +0300 Eli Zaretskii <eliz@gnu.org> wrote:

>> From: Stephen Berman <stephen.berman@gmx.net>
>> Date: Fri, 19 May 2017 11:36:40 +0200
>> 
>> 0. emacs -Q
>> 1. Make a long (> 160 columns) line of text, e.g. in *scratch* join the
>>    two lines and duplicate them: C-p C-p M-^ C-a C-k C-y C-y
>> 2. Copy this long line and yank it below (optional, just to see the
>>    effect of auto-hscrolling clearly): C-a C-k C-y RET C-y
>> 3. M-x toggle-truncate-lines
>> 4. M-x customize-option RET auto-hscroll-mode, set value to "Scroll only
>>    the current line" and set it for the current session.
>> 5. Return to *scratch* and put point at the start: M-<.  Enable
>>    column-number-mode to track the column of point:
>> 6. Start moving point forward, e.g by repeating M-f.  On moving over
>>    the first occurrence of "create", this line (but not the one below
>>    it) is scrolled, correctly.  Continue repeating M-f.
>> => When point reached column 160 (for me that's the space after the
>>    second occurrence of ";; This buffer is"), the cursor is visible, but
>>    the next M-f makes it disappear and the line no longer scrolling;
>>    however, you can see from the column number in the mode line that
>>    point keeps advancing.
>> 
>> When scrolling stops depends, AFAICT, on (one or both of) the
>> window-width and the font size.
>> 
>> A second, likely related, display bug happens in combination with
>> text-scale-adjust:
>> 
>> 7. Return to BOB (M-<).  Invoke increased text-scale-adjust (C-x C-+)
>>    and start moving forward by repeating M-f.
>> => At some point (for me at column 130), while advancing point
>>    columnwise (C-f) the line also moves pixelwise to the left and then
>>    (after 3-4 pixels) jumps back.  Continuing with C-f repeats this
>>    effect.
>>    With decreased text-scale-adjust (C-x C--, making sure the size is
>>    below the default, not just returned to it), the mirror-image effect
>>    happens (for me, starting at column 176 with the smaller size): the
>>    line moves pixelwise to the right and after several pixels jumps back
>>    to the left, and so on.
>>    When the default size is restored, the pixelwise movement stops (but
>>    the above hscrolling problem remains).
>
> Thanks, should be fixed now.

Confirmed, thanks!

Steve Berman