Allow controlling the underline position of faces

* doc/lispref/display.texi (Face Attributes): Document new
`:position' property of the `:underline' attribute.
* etc/NEWS: Announce new property.
* lisp/cus-face.el (custom-face-attributes): Implement
customization for new face attribute.
* src/dispextern.h (struct face): New fields
`underline_pixels_above_descent_line' and
`underline_at_descent_line_p'.
* src/haikuterm.c (haiku_draw_text_decoration):
* src/nsterm.m (ns_draw_text_decoration):
* src/w32term.c (w32_draw_glyph_string):
* src/xterm.c (x_draw_glyph_string): Respect new face fields.

* src/xfaces.c (realize_gui_face): Handle new `:position'
keyword.
(syms_of_xfaces): New symbol `:position'.

6 files changed, 81 insertions, 18 deletions

diff --git a/src/dispextern.h b/src/dispextern.h
index 954992a0ec2..368507732ce 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -1720,6 +1720,12 @@ struct face
   int box_vertical_line_width;
   int box_horizontal_line_width;
 
+
+  /* The amount of pixels above the descent line the underline should
+     be displayed.  It does not take effect unless
+     `underline_at_descent_line_p` is t.  */
+  int underline_pixels_above_descent_line;
+
   /* Type of box drawn.  A value of FACE_NO_BOX means no box is drawn
      around text in this face.  A value of FACE_SIMPLE_BOX means a box
      of width box_line_width is drawn in color box_color.  A value of
@@ -1753,6 +1759,9 @@ struct face
   bool_bf strike_through_color_defaulted_p : 1;
   bool_bf box_color_defaulted_p : 1;
 
+  /* True means the underline should be drawn at the descent line.  */
+  bool_bf underline_at_descent_line_p : 1;
+
   /* TTY appearances.  Colors are found in `lface' with empty color
      string meaning the default color of the TTY.  */
   bool_bf tty_bold_p : 1;
diff --git a/src/haikuterm.c b/src/haikuterm.c
index b6d105bf2b6..2304f718c31 100644
--- a/src/haikuterm.c
+++ b/src/haikuterm.c
@@ -613,7 +613,12 @@ haiku_draw_text_decoration (struct glyph_string *s, struct face *face,
           unsigned long thickness, position;
           int y;
 
-          if (s->prev && s->prev && s->prev->hl == DRAW_MOUSE_FACE)
+          if (s->prev
+              && s->prev->face->underline == FACE_UNDER_LINE
+              && (s->prev->face->underline_at_descent_line_p
+                  == s->face->underline_at_descent_line_p)
+              && (s->prev->face->underline_pixels_above_descent_line
+                  == s->face->underline_pixels_above_descent_line))
             {
               struct face *prev_face = s->prev->face;
 
@@ -644,7 +649,8 @@ haiku_draw_text_decoration (struct glyph_string *s, struct face *face,
               val = (WINDOW_BUFFER_LOCAL_VALUE
                      (Qx_underline_at_descent_line, s->w));
               underline_at_descent_line
-                = !(NILP (val) || EQ (val, Qunbound));
+                = (!(NILP (val) || EQ (val, Qunbound))
+                   || s->face->underline_at_descent_line_p);
 
               val = (WINDOW_BUFFER_LOCAL_VALUE
                      (Qx_use_underline_position_properties, s->w));
@@ -657,7 +663,9 @@ haiku_draw_text_decoration (struct glyph_string *s, struct face *face,
               else
                 thickness = 1;
               if (underline_at_descent_line)
-                position = (s->height - thickness) - (s->ybase - s->y);
+                position = ((s->height - thickness)
+                            - (s->ybase - s->y)
+                            - s->face->underline_pixels_above_descent_line);
               else
                 {
                   /* Get the underline position.  This is the
diff --git a/src/nsterm.m b/src/nsterm.m
index a15dc47a226..4f60cc737da 100644
--- a/src/nsterm.m
+++ b/src/nsterm.m
@@ -3265,7 +3265,11 @@ ns_draw_text_decoration (struct glyph_string *s, struct face *face,
           /* If the prev was underlined, match its appearance.  */
           if (s->prev
               && s->prev->face->underline == FACE_UNDER_LINE
-              && s->prev->underline_thickness > 0)
+              && s->prev->underline_thickness > 0
+              && (s->prev->face->underline_at_descent_line_p
+                  == s->face->underline_at_descent_line_p)
+              && (s->prev->face->underline_pixels_above_descent_line
+                  == s->face->underline_pixels_above_descent_line))
             {
               thickness = s->prev->underline_thickness;
               position = s->prev->underline_position;
@@ -3286,7 +3290,8 @@ ns_draw_text_decoration (struct glyph_string *s, struct face *face,
 
               val = (WINDOW_BUFFER_LOCAL_VALUE
                      (Qx_underline_at_descent_line, s->w));
-              underline_at_descent_line = !(NILP (val) || EQ (val, Qunbound));
+              underline_at_descent_line = (!(NILP (val) || EQ (val, Qunbound))
+                                           || s->face->underline_at_descent_line_p);
 
               val = (WINDOW_BUFFER_LOCAL_VALUE
                      (Qx_use_underline_position_properties, s->w));
@@ -3299,7 +3304,8 @@ ns_draw_text_decoration (struct glyph_string *s, struct face *face,
 
               /* Determine the offset of underlining from the baseline.  */
               if (underline_at_descent_line)
-                position = descent - thickness;
+                position = (descent - thickness
+                            - s->face->underline_pixels_above_descent_line);
               else if (use_underline_position_properties
                        && font && font->underline_position >= 0)
                 position = font->underline_position;
@@ -3308,7 +3314,8 @@ ns_draw_text_decoration (struct glyph_string *s, struct face *face,
               else
                 position = minimum_offset;
 
-              position = max (position, minimum_offset);
+              if (!s->face->underline_pixels_above_descent_line)
+                position = max (position, minimum_offset);
 
               /* Ensure underlining is not cropped.  */
               if (descent <= position)
diff --git a/src/w32term.c b/src/w32term.c
index 700c492cc37..78777f153c0 100644
--- a/src/w32term.c
+++ b/src/w32term.c
@@ -2564,7 +2564,11 @@ w32_draw_glyph_string (struct glyph_string *s)
               int y;
 
               if (s->prev
-                  && s->prev->face->underline == FACE_UNDER_LINE)
+                  && s->prev->face->underline == FACE_UNDER_LINE
+                  && (s->prev->face->underline_at_descent_line_p
+                      == s->face->underline_at_descent_line_p)
+                  && (s->prev->face->underline_pixels_above_descent_line
+                      == s->face->underline_pixels_above_descent_line))
                 {
                   /* We use the same underline style as the previous one.  */
                   thickness = s->prev->underline_thickness;
@@ -2587,7 +2591,8 @@ w32_draw_glyph_string (struct glyph_string *s)
                   val = (WINDOW_BUFFER_LOCAL_VALUE
                          (Qx_underline_at_descent_line, s->w));
                   underline_at_descent_line
-                    = !(NILP (val) || EQ (val, Qunbound));
+                    = (!(NILP (val) || EQ (val, Qunbound))
+                       || s->face->underline_at_descent_line_p);
 
                   val = (WINDOW_BUFFER_LOCAL_VALUE
                          (Qx_use_underline_position_properties, s->w));
@@ -2601,7 +2606,9 @@ w32_draw_glyph_string (struct glyph_string *s)
                     thickness = 1;
                   if (underline_at_descent_line
                       || !font)
-                    position = (s->height - thickness) - (s->ybase - s->y);
+                    position = ((s->height - thickness)
+                                - (s->ybase - s->y)
+                                - s->face->underline_pixels_above_descent_line);
                   else
                     {
                       /* Get the underline position.  This is the
@@ -2619,7 +2626,12 @@ w32_draw_glyph_string (struct glyph_string *s)
                       else
                         position = (font->descent + 1) / 2;
                     }
-                  position = max (position, minimum_offset);
+
+                  if (!(s->face->underline_at_descent_line_p
+                        /* Ignore minimum_offset if the amount of pixels
+                           was explictly specified.  */
+                        && s->face->underline_pixels_above_descent_line))
+                    position = max (position, minimum_offset);
                 }
               /* Check the sanity of thickness and position.  We should
                  avoid drawing underline out of the current line area.  */
diff --git a/src/xfaces.c b/src/xfaces.c
index 3fd31b7f225..8064d47c947 100644
--- a/src/xfaces.c
+++ b/src/xfaces.c
@@ -6041,6 +6041,8 @@ realize_gui_face (struct face_cache *cache, Lisp_Object attrs[LFACE_VECTOR_SIZE]
       face->underline = FACE_UNDER_LINE;
       face->underline_defaulted_p = true;
       face->underline_color = 0;
+      face->underline_at_descent_line_p = false;
+      face->underline_pixels_above_descent_line = 0;
     }
   else if (STRINGP (underline))
     {
@@ -6050,12 +6052,16 @@ realize_gui_face (struct face_cache *cache, Lisp_Object attrs[LFACE_VECTOR_SIZE]
       face->underline_color
         = load_color (f, face, underline,
                       LFACE_UNDERLINE_INDEX);
+      face->underline_at_descent_line_p = false;
+      face->underline_pixels_above_descent_line = 0;
     }
   else if (NILP (underline))
     {
       face->underline = FACE_NO_UNDERLINE;
       face->underline_defaulted_p = false;
       face->underline_color = 0;
+      face->underline_at_descent_line_p = false;
+      face->underline_pixels_above_descent_line = 0;
     }
   else if (CONSP (underline))
     {
@@ -6064,6 +6070,8 @@ realize_gui_face (struct face_cache *cache, Lisp_Object attrs[LFACE_VECTOR_SIZE]
       face->underline = FACE_UNDER_LINE;
       face->underline_color = 0;
       face->underline_defaulted_p = true;
+      face->underline_at_descent_line_p = false;
+      face->underline_pixels_above_descent_line = 0;
 
       /* FIXME?  This is also not robust about checking the precise form.
          See comments in Finternal_set_lisp_face_attribute.  */
@@ -6100,6 +6108,13 @@ realize_gui_face (struct face_cache *cache, Lisp_Object attrs[LFACE_VECTOR_SIZE]
               else if (EQ (value, Qwave))
                 face->underline = FACE_UNDER_WAVE;
             }
+          else if (EQ (keyword, QCposition))
+            {
+              face->underline_at_descent_line_p = !NILP (value);
+
+              if (FIXNATP (value))
+                face->underline_pixels_above_descent_line = XFIXNAT (value);
+            }
         }
     }
 
@@ -6915,6 +6930,7 @@ syms_of_xfaces (void)
   DEFSYM (QCcolor, ":color");
   DEFSYM (QCline_width, ":line-width");
   DEFSYM (QCstyle, ":style");
+  DEFSYM (QCposition, ":position");
   DEFSYM (Qline, "line");
   DEFSYM (Qwave, "wave");
   DEFSYM (Qreleased_button, "released-button");
diff --git a/src/xterm.c b/src/xterm.c
index 321b4c5ab66..e2b09938a27 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -4144,8 +4144,12 @@ x_draw_glyph_string (struct glyph_string *s)
               unsigned long thickness, position;
               int y;
 
-              if (s->prev &&
-                  s->prev->face->underline == FACE_UNDER_LINE)
+              if (s->prev
+                  && s->prev->face->underline == FACE_UNDER_LINE
+                  && (s->prev->face->underline_at_descent_line_p
+                      == s->face->underline_at_descent_line_p)
+                  && (s->prev->face->underline_pixels_above_descent_line
+                      == s->face->underline_pixels_above_descent_line))
                 {
                   /* We use the same underline style as the previous one.  */
                   thickness = s->prev->underline_thickness;
@@ -4168,7 +4172,8 @@ x_draw_glyph_string (struct glyph_string *s)
                   val = (WINDOW_BUFFER_LOCAL_VALUE
                          (Qx_underline_at_descent_line, s->w));
                   underline_at_descent_line
-                    = !(NILP (val) || EQ (val, Qunbound));
+                    = (!(NILP (val) || EQ (val, Qunbound))
+                       || s->face->underline_at_descent_line_p);
 
                   val = (WINDOW_BUFFER_LOCAL_VALUE
                          (Qx_use_underline_position_properties, s->w));
@@ -4181,7 +4186,9 @@ x_draw_glyph_string (struct glyph_string *s)
                   else
                     thickness = 1;
                   if (underline_at_descent_line)
-                    position = (s->height - thickness) - (s->ybase - s->y);
+                    position = ((s->height - thickness)
+                                - (s->ybase - s->y)
+                                - s->face->underline_pixels_above_descent_line);
                   else
                     {
                       /* Get the underline position.  This is the
@@ -4201,12 +4208,16 @@ x_draw_glyph_string (struct glyph_string *s)
                       else
                         position = minimum_offset;
                     }
-                  position = max (position, minimum_offset);
+
+                  /* Ignore minimum_offset if the amount of pixels was
+                     explictly specified.  */
+                  if (!s->face->underline_pixels_above_descent_line)
+                    position = max (position, minimum_offset);
                 }
               /* Check the sanity of thickness and position.  We should
                  avoid drawing underline out of the current line area.  */
-              if (s->y + s->height <= s->ybase + position)
-                position = (s->height - 1) - (s->ybase - s->y);
+              if (s->y + s->height <= s->ybase + position)
+                position = (s->height - 1) - (s->ybase - s->y);
               if (s->y + s->height < s->ybase + position + thickness)
                 thickness = (s->y + s->height) - (s->ybase + position);
               s->underline_thickness = thickness;