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

[Stephen Berman <stephen.berman@gmx.net>]

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).

In GNU Emacs 26.0.50 (build 7, x86_64-pc-linux-gnu, GTK+ Version 3.22.8)
 of 2017-05-19 built on rosalinde
Repository revision: bb5c6614eb171eef8c08474e0f949fe18ef4b230
Windowing system distributor 'The X.Org Foundation', version 11.0.11901000
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