From: Eli Zaretskii <eliz@gnu.org>
Date: Sat, 7 Nov 2020 08:27:15 +0000 (+0200)
Subject: Fix scrolling problems with misc-fixed fonts under Cairo
X-Git-Tag: emacs-28.0.90~5217
X-Git-Url: http://git.eshelyaron.com/gitweb/?a=commitdiff_plain;h=33e2418a7cfd2ac1b98b86c5ddaf99c1d90daaf0;p=emacs.git

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 a10308c62ee..b89510704e1 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);
     }