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

[Eli Zaretskii <eliz@gnu.org>]

> 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.