bug#28605: 26.0.60; Part of leftmost character hidden

[martin rudalics]

> If the frame contains three side-by-side windows and scroll bars on
> the right I expect to see (from left to right):
>
> frame border
> left fringe for window one
> window one
> right fringe for window one
> scroll bar for window one
> right fringe for window two
> window two
> left fringe for window two
> scroll bar for window two
> right fringe for window three
> window three
> left fringe  for window three
> scroll bar for window three
> frame border
>
> Of all the fringes, only the left fringe of window one is visible.
> Using scroll bars on the left, only the right fringe of window three
> is visible.
> In both cases, characters that are next to non-visible/hidden fringes
> are partially hidden.
> I'm attaching two screenshots that should show this.

Thanks for the details.  From your initial screenshot it was not clear
to me that the effect is really that bad.

> I bit the bullet and did a git bisect:
>
> 36cf0791ba75ee16dfbedfe437567ec6dd945b8a is the first bad commit
> commit 36cf0791ba75ee16dfbedfe437567ec6dd945b8a
> Author: Lars Ingebrigtsen <larsi@gnus.org>
> Date:   Sun Jul 16 16:50:57 2017 +0200
>      Remove usage of the GDK_SCALE variable
>
>      * src/gtkutil.c (xg_get_gdk_scale): Remove.
>      (xg_get_default_scrollbar_height)
>      (xg_get_default_scrollbar_width): Pass in a frame to check for
>      scaling.
>      (xg_frame_set_char_size): Use the API for querying scale
>      instead of looking at the GDK_SCALE variable.
>      (xg_get_default_scrollbar_width): Ditto.
>      (xg_get_default_scrollbar_height): Ditto.
>      (xg_update_scrollbar_pos): Ditto.
>
>      * src/xfns.c (x_set_scroll_bar_default_height): Pass in the
>      frame to get the width.

Great.  Lars, how should we proceed from here?

(1) Revert that commit.

(2) Revert the call of gtk_widget_get_scale_factor only.

(3) Try to fix the build with your change in place.

(4) Provide some option so that users can either use your approach or
    Jan's.  The default should prefer Jan's setup to avoid that users
    like Ola have to go through this again.

> I also played with the dividers and horizontal scroll bars.
>
> The horizontal scroll bars are not visibla at all with scaling on.

Apparently, the horizontal scroll bar code does not handle scaling.  If
we fix the current problem, maybe you could help me fix that one too.

> Dividers on the right fixed the hidden character problem but not the
> hidden fringes,

Yes.  IMHO it's because the scroll bar clearing code hides the dividers
instead of the text.  If you customize dividers to a sufficiently large
width you should be able to make the entire text visible and even parts
of the dividers (on the left side of a window, only - the right sides
will remain obscured as before).

> I attach some screenshots of this as well.

Very informative, thanks.  One thing that stupefies me about these
screenshots is that the menu and toolbar items are smaller in
horizontal-scrollbars-no-dividers.png than in with-hidpi-scaling.png.

> This is with emacs -Q --no-x-resources, dividers off, horizontal scrollbars 
off.
>
> frame pixel: 2034 x 466   cols/lines: 226 x 25   units: 9 x 18
[...]
> height header-line: 0  mode-line: 0  divider: 0

All these values are completely as expected so the problem must be
confined to this x_clear_area call in xg_update_scrollbar_pos in
gtkutil.c

          /* Clear under old scroll bar position.  */
          oldw += (scale - 1) * oldw;
          oldx -= (scale - 1) * oldw;
          x_clear_area (f, oldx, oldy, oldw, oldh);

where the value of scale is now obtained via

      int scale = xg_get_scale (f);

If you replace the latter by the old

      int scale = xg_get_gdk_scale ();

does your problem go away?  If so, then it would be interesting how the
values returned by xg_get_scale differ with gtk_widget_get_scale_factor
and xg_get_gdk_scale called.

Thanks, martin

CC-ing Kaushal Modi: Is this the same problem as the one you posted in
Bug#27830?