Bug#972572: emacs-gtk: scrolling upward with the mouse wheel actually scrolls downward when the cursor needs repositioning

[Vincent Lefevre]

Control: tags -1 fixed-upstream patch

On 2020-11-01 01:37:18 +0100, Vincent Lefevre wrote:
> The cause is that with the cairo version (like in Debian),
> row->phys_ascent > row->ascent. There are 2 issues:
> 
> 1. The fact that this condition is true with misc-fixed 13-pixel fonts.

I've eventually found the root cause: a ceil() is applied on a value
that is expected to be an integer here (this is a bitmap font, thus
with integer parameters), but due to a rounding error in Cairo, one
gets a slightly larger value, so that ceil() gives the next integer:

  https://debbugs.gnu.org/cgi/bugreport.cgi?bug=44284#74

Note that this explains why the issue occurs only with a particular
font size (the rounding error depends on the font size, and the issue
can occur only when this error is positive and large enough).

I've suggested several solutions:

  https://debbugs.gnu.org/cgi/bugreport.cgi?bug=44284#80

In short: Cairo should ideally produce a correctly rounded value,
but guaranteeing that in general may be complex and may yield a
performance drop. Alternatively, assume in Emacs that a value
slightly above the integer is this integer. That's how this has
been patched by Eli Zaretskii:

-      cache->ascent = ceil (- extents.y_bearing);
+      cache->ascent = ceil (- extents.y_bearing - 1.0 / 256);

(see attached patch, which applies to the Debian version too).

> 2. The fact that this condition is handled incorrectly by Emacs.

Thanks to the above patch, this condition no longer occurs in this
case. I don't know whether there are issues when it occurs in other
cases.

-- 
Vincent Lefèvre <vinc...@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

commit 33e2418a7cfd2ac1b98b86c5ddaf99c1d90daaf0
Author: Eli Zaretskii <e...@gnu.org>
Date:   2020-11-07 09:27:15 +0100

    Fix scrolling problems with misc-fixed fonts under Cairo
    
    * src/ftcrfont.c (ftcrfont_glyph_extents): Avoid rounding up the
    glyph ascent to a higher value than needed due to floating-point
    roundoff errors.  (Bug#44284)

diff --git a/src/ftcrfont.c b/src/ftcrfont.c
index a10308c62e..b89510704e 100644
--- a/src/ftcrfont.c
+++ b/src/ftcrfont.c
@@ -84,7 +84,12 @@ ftcrfont_glyph_extents (struct font *font,
       cache->lbearing = floor (extents.x_bearing);
       cache->rbearing = ceil (extents.width + extents.x_bearing);
       cache->width = lround (extents.x_advance);
-      cache->ascent = ceil (- extents.y_bearing);
+      /* The subtraction of a small number is to avoid rounding up due
+        to floating-point inaccuracies with some fonts, which then
+        could cause unpleasant effects while scrolling (see bug
+        #44284), since we then think that a glyph row's ascent is too
+        small to accommodate a glyph with a higher phys_ascent.  */
+      cache->ascent = ceil (- extents.y_bearing - 1.0 / 256);
       cache->descent = ceil (extents.height + extents.y_bearing);
     }