A better fix for bug#25845

* src/xdisp.c (font_for_underline_metrics): New function.
* src/dispextern.h: Add its prototype.
* src/xterm.c (x_draw_glyph_string):
* src/w32term.c (x_draw_glyph_string):
* src/nsterm.m (ns_draw_text_decoration): Call it.  This avoids
having identical code 3 times in 3 different files.

5 files changed, 34 insertions, 64 deletions

diff --git a/src/dispextern.h b/src/dispextern.h
index e030618a9b7..679820d5063 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -3293,6 +3293,7 @@ extern void dump_glyph_string (struct glyph_string *) EXTERNALLY_VISIBLE;
 
 extern void x_get_glyph_overhangs (struct glyph *, struct frame *,
                                    int *, int *);
+extern struct font *font_for_underline_metrics (struct glyph_string *);
 extern void x_produce_glyphs (struct it *);
 
 extern void x_write_glyphs (struct window *, struct glyph_row *,
diff --git a/src/nsterm.m b/src/nsterm.m
index bc89925b0ef..08ee0cdf6fd 100644
--- a/src/nsterm.m
+++ b/src/nsterm.m
@@ -3043,28 +3043,7 @@ ns_draw_text_decoration (struct glyph_string *s, struct face *face,
             }
           else
             {
-              /* If we are drawing in the middle of a glyph row, find
-                 the first glyph in the run of underlined glyphs
-                 preceding the beginning of glyph string S.  This is
-                 because that glyph determines the underline position
-                 and thickness for the entire run of the underlined
-                 glyphs.  */
-              struct glyph *g0 = s->row->glyphs[s->area], *g;
-
-              for (g = s->first_glyph - 1; g >= g0; g--)
-                {
-                  struct face *prev_face = FACE_FROM_ID (s->f, g->face_id);
-                  if (!(prev_face && prev_face->underline_p))
-                    break;
-                }
-
-              /* Now use the font of the last glyph we saw that
-                 still has the underlined_p flag set.  */
-              struct face *glyph_face = FACE_FROM_ID (s->f, g[1].face_id);
-              struct font *font = glyph_face->font;
-              if (font)
-                font_prepare_for_face (s->f, glyph_face);
-
+              struct font *font = font_for_underline_metrics (s);
               unsigned long descent = s->y + s->height - s->ybase;
 
               /* Use underline thickness of font, defaulting to 1. */
diff --git a/src/w32term.c b/src/w32term.c
index 6a98fc721cd..81666f5bc47 100644
--- a/src/w32term.c
+++ b/src/w32term.c
@@ -2433,27 +2433,7 @@ x_draw_glyph_string (struct glyph_string *s)
                 }
               else
                 {
-                  /* If we are drawing in the middle of a glyph row,
-                     find the first glyph in the run of underlined
-                     glyphs preceding the beginning of glyph string S.
-                     This is because that glyph determines the
-                     underline position and thickness for the entire
-                     run of the underlined glyphs.  */
-                  struct glyph *g0 = s->row->glyphs[s->area], *g;
-
-                  for (g = s->first_glyph - 1; g >= g0; g--)
-                    {
-                      struct face *prev_face = FACE_FROM_ID (s->f, g->face_id);
-                      if (!(prev_face && prev_face->underline_p))
-                        break;
-                    }
-
-                  /* Now use the font of the last glyph we saw that
-                     still has the underlined_p flag set.  */
-                  struct face *glyph_face = FACE_FROM_ID (s->f, g[1].face_id);
-                  struct font *font = glyph_face->font;
-                  if (font)
-                    font_prepare_for_face (s->f, glyph_face);
+                  struct font *font = font_for_underline_metrics (s);
 
                   /* Get the underline thickness.  Default is 1 pixel.  */
                   if (font && font->underline_thickness > 0)
diff --git a/src/xdisp.c b/src/xdisp.c
index 82c4c775c16..1e7cb4ec665 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -25948,6 +25948,36 @@ draw_glyphs (struct window *w, int x, struct glyph_row *row,
   return x_reached;
 }
 
+/* Find the first glyph in the run of underlined glyphs preceding the
+   beginning of glyph string S, and return its font (which could be
+   NULL).  This is needed because that font determines the underline
+   position and thickness for the entire run of the underlined glyphs.
+   This function is called from the draw_glyph_string method of GUI
+   frame's redisplay interface (RIF) when it needs to draw in an
+   underlined face.  */
+struct font *
+font_for_underline_metrics (struct glyph_string *s)
+{
+  struct glyph *g0 = s->row->glyphs[s->area], *g;
+
+  for (g = s->first_glyph - 1; g >= g0; g--)
+    {
+      struct face *prev_face = FACE_FROM_ID (s->f, g->face_id);
+      if (!(prev_face && prev_face->underline_p))
+        break;
+    }
+
+  /* If preceding glyphs are not underlined, use the font of S.  */
+  if (g == s->first_glyph - 1)
+    return s->font;
+  else
+    {
+      /* Otherwise use the font of the last glyph we saw in the above
+         loop whose face had the underline_p flag set.  */
+      return FACE_FROM_ID (s->f, g[1].face_id)->font;
+    }
+}
+
 /* Expand row matrix if too narrow.  Don't expand if area
    is not present.  */
 
diff --git a/src/xterm.c b/src/xterm.c
index 57e64c48887..28faea14a3a 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -3636,27 +3636,7 @@ x_draw_glyph_string (struct glyph_string *s)
                 }
               else
                 {
-                  /* If we are drawing in the middle of a glyph row,
-                     find the first glyph in the run of underlined
-                     glyphs preceding the beginning of glyph string S.
-                     This is because that glyph determines the
-                     underline position and thickness for the entire
-                     run of the underlined glyphs.  */
-                  struct glyph *g0 = s->row->glyphs[s->area], *g;
-
-                  for (g = s->first_glyph - 1; g >= g0; g--)
-                    {
-                      struct face *prev_face = FACE_FROM_ID (s->f, g->face_id);
-                      if (!(prev_face && prev_face->underline_p))
-                        break;
-                    }
-
-                  /* Now use the font of the last glyph we saw that
-                     still has the underlined_p flag set.  */
-                  struct face *glyph_face = FACE_FROM_ID (s->f, g[1].face_id);
-                  struct font *font = glyph_face->font;
-                  if (font)
-                    font_prepare_for_face (s->f, glyph_face);
+                  struct font *font = font_for_underline_metrics (s);
 
                   /* Get the underline thickness.  Default is 1 pixel.  */
                   if (font && font->underline_thickness > 0)