Fix display of cursor on underlined text

* src/nsterm.m (ns_draw_text_decoration):
* src/xterm.c (x_draw_glyph_string):
* src/w32term.c (x_draw_glyph_string): Compute the position and
thickness of the underline by looking for the first glyph of the
run of underlined glyphs that includes the glyph string we are
drawing.  (Bug#25845)

1 files changed, 28 insertions, 6 deletions

diff --git a/src/xterm.c b/src/xterm.c
index 24d1702cec0..57e64c48887 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -3636,9 +3636,31 @@ 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);
+
                   /* Get the underline thickness.  Default is 1 pixel.  */
-                  if (s->font && s->font->underline_thickness > 0)
-                    thickness = s->font->underline_thickness;
+                  if (font && font->underline_thickness > 0)
+                    thickness = font->underline_thickness;
                   else
                     thickness = 1;
                   if (x_underline_at_descent_line)
@@ -3654,10 +3676,10 @@ x_draw_glyph_string (struct glyph_string *s)
                          ROUND(x) = floor (x + 0.5)  */
 
                       if (x_use_underline_position_properties
-                          && s->font && s->font->underline_position >= 0)
-                        position = s->font->underline_position;
-                      else if (s->font)
-                        position = (s->font->descent + 1) / 2;
+                          && font && font->underline_position >= 0)
+                        position = font->underline_position;
+                      else if (font)
+                        position = (font->descent + 1) / 2;
                       else
                         position = underline_minimum_offset;
                     }