Support cons cell for 'line-spacing'

* etc/NEWS: Announce the change.
* src/dispextern.h (struct glyph_row): Add
'extra_line_spacing_above' member.
(struct it): Add 'extra_line_spacing_above' member.
* src/frame.h (struct frame): Add 'extra_line_spacing_above'
member.  Update comment for 'extra_line_spacing.'
* src/buffer.c (syms_of_buffer): Update the docstring of
'line-spacing' to describe the cons cell usage.
* src/buffer.h (struct buffer): Update comment for
'extra_line_spacing'.
* src/frame.c (gui_set_line_spacing): Handle cons cell value for
'line-spacing'.  Calculate and set 'extra_line_spacing_above'
for both integer and float pairs.
* src/xdisp.c (init_iterator): Initialize 'extra_line_spacing_above'
from buffer or frame 'line-spacing', handling cons cells for both
integer and float values.
(gui_produce_glyphs): Use 'extra_line_spacing_above' to distribute
spacing between ascent and descent.  Update 'max_extra_line_spacing'
calculation.
(resize_mini_window): Take line spacing into account when resizing the
mini window.  Pass height of a single line to 'grow_mini_window' and
'shrink_mini_window'.
* src/window.c (grow_mini_window, shrink_mini_window): Add unit
argument which defines height of a single line.
* src/window.h (grow_mini_window, shrink_mini_window): Adjust function
prototypes accordingly with unit argument.
* lisp/subr.el (total-line-spacing): New function to calculate total
spacing from a number or cons cell.
(posn-col-row): Use total-line-spacing.
* lisp/simple.el (default-line-height): Use 'total-line-spacing'.
* lisp/textmodes/picture.el (picture-mouse-set-point): Use
'total-line-spacing'.
* lisp/window.el (window-default-line-height): Use
'total-line-spacing'.
(window--resize-mini-window): Take 'line-spacing' into account.
* test/lisp/subr-tests.el (total-line-spacing): New test.
* test/src/buffer-tests.el (test-line-spacing): New test.
* doc/emacs/display.texi (Display Custom): Document that
'line-spacing' can be a cons cell.
(Line Height): Document the new cons cell format for 'line-spacing'
to allow vertical centering.

Co-authored-by: Przemysław Alexander Kamiński <alexander@kaminski.se>
Co-authored-by: Daniel Mendler <mail@daniel-mendler.de>

8 files changed, 140 insertions, 30 deletions

diff --git a/src/buffer.c b/src/buffer.c
index fec30d4f0e9..3d85d784f1c 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -5875,12 +5875,15 @@ cursor's appearance is instead controlled by the variable
 `cursor-in-non-selected-windows'.  */);
 
   DEFVAR_PER_BUFFER ("line-spacing",
-                     &BVAR (current_buffer, extra_line_spacing), Qnumberp,
+                     &BVAR (current_buffer, extra_line_spacing), Qnil,
                      doc: /* Additional space to put between lines when displaying a buffer.
 The space is measured in pixels, and put below lines on graphic displays,
 see `display-graphic-p'.
 If value is a floating point number, it specifies the spacing relative
-to the default frame line height.  A value of nil means add no extra space.  */);
+to the default frame line height.
+If value is a cons cell containing a pair of floats or integers,
+it is interpreted as space above and below the line, respectively.
+A value of nil means add no extra space.  */);
 
   DEFVAR_PER_BUFFER ("cursor-in-non-selected-windows",
                      &BVAR (current_buffer, cursor_in_non_selected_windows), Qnil,
diff --git a/src/buffer.h b/src/buffer.h
index 34fe308b671..403b249df6f 100644
--- a/src/buffer.h
+++ b/src/buffer.h
@@ -575,7 +575,10 @@ struct buffer
   Lisp_Object cursor_type_;
 
   /* An integer > 0 means put that number of pixels below text lines
-     in the display of this buffer.  */
+     in the display of this buffer.
+     A float ~ 1.0 means add extra number of pixels below text lines
+     relative to the line height.
+     A cons means put car spacing above and cdr spacing below the line.  */
   Lisp_Object extra_line_spacing_;
 
 #ifdef HAVE_TREE_SITTER
diff --git a/src/dispextern.h b/src/dispextern.h
index d325e185c5a..30785b9ccdf 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -960,6 +960,9 @@ struct glyph_row
      in last row when checking if row is fully visible.  */
   int extra_line_spacing;
 
+  /* Part of extra_line_spacing that should go above the line.  */
+  int extra_line_spacing_above;
+
   /* First position in this row.  This is the text position, including
      overlay position information etc, where the display of this row
      started, and can thus be less than the position of the first
@@ -2772,6 +2775,10 @@ struct it
      window systems only.)  */
   int extra_line_spacing;
 
+  /* Default amount of additional space in pixels above lines (for
+     window systems only).  */
+  int extra_line_spacing_above;
+
   /* Max extra line spacing added in this row.  */
   int max_extra_line_spacing;
 
diff --git a/src/frame.c b/src/frame.c
index 033215a76ec..ba342b15723 100644
--- a/src/frame.c
+++ b/src/frame.c
@@ -5454,18 +5454,60 @@ void
 gui_set_line_spacing (struct frame *f, Lisp_Object new_value, Lisp_Object old_value)
 {
   if (NILP (new_value))
-    f->extra_line_spacing = 0;
+    {
+      f->extra_line_spacing = 0;
+      f->extra_line_spacing_above = 0;
+    }
   else if (RANGED_FIXNUMP (0, new_value, INT_MAX))
-    f->extra_line_spacing = XFIXNAT (new_value);
+    {
+      f->extra_line_spacing = XFIXNAT (new_value);
+      f->extra_line_spacing_above = 0;
+    }
   else if (FLOATP (new_value))
     {
-      int new_spacing = XFLOAT_DATA (new_value) * FRAME_LINE_HEIGHT (f) + 0.5;
+      int new_spacing = XFLOAT_DATA (new_value) * FRAME_LINE_HEIGHT (f);
 
-      if (new_spacing >= 0)
+      if (new_spacing >= 0) {
         f->extra_line_spacing = new_spacing;
+        f->extra_line_spacing_above = 0;
+      }
       else
         signal_error ("Invalid line-spacing", new_value);
     }
+  else if (CONSP (new_value))
+    {
+      Lisp_Object above = XCAR (new_value);
+      Lisp_Object below = XCDR (new_value);
+
+      /* Integer pair case.  */
+      if (RANGED_FIXNUMP (0, above, INT_MAX)
+          && RANGED_FIXNUMP (0, below, INT_MAX))
+        {
+          f->extra_line_spacing = XFIXNAT (above) + XFIXNAT (below);
+          f->extra_line_spacing_above = XFIXNAT (above);
+        }
+
+      /* Float pair case.  */
+      else if (FLOATP (XCAR (new_value))
+               && FLOATP (XCDR (new_value)))
+        {
+          int new_spacing = (XFLOAT_DATA (above) + XFLOAT_DATA (below)) * FRAME_LINE_HEIGHT (f);
+          int spacing_above = XFLOAT_DATA (above) * FRAME_LINE_HEIGHT (f);
+          if(new_spacing >= 0 && spacing_above >= 0)
+            {
+              f->extra_line_spacing = new_spacing;
+              f->extra_line_spacing_above = spacing_above;
+            }
+          else
+            signal_error ("Invalid line-spacing", new_value);
+        }
+
+      /* Unmatched pair case.  */
+      else
+        {
+          signal_error ("Invalid line-spacing", new_value);
+        }
+    }
   else
     signal_error ("Invalid line-spacing", new_value);
   if (FRAME_VISIBLE_P (f))
diff --git a/src/frame.h b/src/frame.h
index c369a848b7c..091b112e8b9 100644
--- a/src/frame.h
+++ b/src/frame.h
@@ -718,9 +718,16 @@ struct frame
      frame parameter.  0 means don't do gamma correction.  */
   double gamma;
 
-  /* Additional space to put between text lines on this frame.  */
+  /* Additional space to put below text lines on this frame.
+     Also takes part in line height calculation.  */
   int extra_line_spacing;
 
+  /* Amount of space (included in extra_line_spacing) that goes ABOVE
+     line line.
+     IMPORTANT: Don't use this for line height calculations.
+                (5 . 20) means that extra_line_spacing is 25 with 5 above.  */
+  int extra_line_spacing_above;
+
   /* All display backends seem to need these two pixel values.  */
   unsigned long background_pixel;
   unsigned long foreground_pixel;
diff --git a/src/window.c b/src/window.c
index 497c587b167..c4f2e4e491f 100644
--- a/src/window.c
+++ b/src/window.c
@@ -5894,11 +5894,11 @@ resize_mini_window_apply (struct window *w, int delta)
  * line of text.
  */
 void
-grow_mini_window (struct window *w, int delta)
+grow_mini_window (struct window *w, int delta, int unit)
 {
   struct frame *f = XFRAME (w->frame);
   int old_height = window_body_height (w, WINDOW_BODY_IN_PIXELS);
-  int min_height = FRAME_LINE_HEIGHT (f);
+  int min_height = unit;
 
   eassert (MINI_WINDOW_P (w));
 
@@ -5926,7 +5926,7 @@ grow_mini_window (struct window *w, int delta)
         resize_mini_window_apply (w, -XFIXNUM (grow));
     }
   FRAME_WINDOWS_FROZEN (f)
-    = window_body_height (w, WINDOW_BODY_IN_PIXELS) > FRAME_LINE_HEIGHT (f);
+    = window_body_height (w, WINDOW_BODY_IN_PIXELS) > unit;
 }
 
 /**
@@ -5936,11 +5936,10 @@ grow_mini_window (struct window *w, int delta)
  * line of text.
  */
 void
-shrink_mini_window (struct window *w)
+shrink_mini_window (struct window *w, int unit)
 {
   struct frame *f = XFRAME (w->frame);
-  int delta = (window_body_height (w, WINDOW_BODY_IN_PIXELS)
-               - FRAME_LINE_HEIGHT (f));
+  int delta = (window_body_height (w, WINDOW_BODY_IN_PIXELS) - unit);
 
   eassert (MINI_WINDOW_P (w));
 
@@ -5959,10 +5958,10 @@ shrink_mini_window (struct window *w)
   else if (delta < 0)
     /* delta can be less than zero after adding horizontal scroll
        bar.  */
-    grow_mini_window (w, -delta);
+    grow_mini_window (w, -delta, unit);
 
   FRAME_WINDOWS_FROZEN (f)
-    = window_body_height (w, WINDOW_BODY_IN_PIXELS) > FRAME_LINE_HEIGHT (f);
+    = window_body_height (w, WINDOW_BODY_IN_PIXELS) > unit;
 }
 
 DEFUN ("resize-mini-window-internal", Fresize_mini_window_internal,
diff --git a/src/window.h b/src/window.h
index 5a75f62cc6e..1b4939b40a6 100644
--- a/src/window.h
+++ b/src/window.h
@@ -1126,8 +1126,8 @@ extern Lisp_Object window_from_coordinates (struct frame *, int, int,
 extern void resize_frame_windows (struct frame *, int, bool);
 extern void restore_window_configuration (Lisp_Object);
 extern void delete_all_child_windows (Lisp_Object);
-extern void grow_mini_window (struct window *, int);
-extern void shrink_mini_window (struct window *);
+extern void grow_mini_window (struct window *, int, int);
+extern void shrink_mini_window (struct window *, int);
 extern int window_relative_x_coord (struct window *, enum window_part, int);
 
 void run_window_change_functions (void);
diff --git a/src/xdisp.c b/src/xdisp.c
index a295131a311..fa826c366dd 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -3316,13 +3316,50 @@ init_iterator (struct it *it, struct window *w,
   if (base_face_id == DEFAULT_FACE_ID
       && FRAME_WINDOW_P (it->f))
     {
+      Lisp_Object line_space_above;
+      Lisp_Object line_space_below;
+
       if (FIXNATP (BVAR (current_buffer, extra_line_spacing)))
-        it->extra_line_spacing = XFIXNAT (BVAR (current_buffer, extra_line_spacing));
+        {
+          it->extra_line_spacing = XFIXNAT (BVAR (current_buffer, extra_line_spacing));
+          it->extra_line_spacing_above = 0;
+        }
       else if (FLOATP (BVAR (current_buffer, extra_line_spacing)))
-        it->extra_line_spacing = (XFLOAT_DATA (BVAR (current_buffer, extra_line_spacing))
-                                  * FRAME_LINE_HEIGHT (it->f));
+        {
+          it->extra_line_spacing = (XFLOAT_DATA (BVAR (current_buffer, extra_line_spacing))
+                                    * FRAME_LINE_HEIGHT (it->f));
+          it->extra_line_spacing_above = 0;
+        }
+      else if (CONSP (BVAR (current_buffer, extra_line_spacing)))
+        {
+          line_space_above = XCAR (BVAR (current_buffer, extra_line_spacing));
+          line_space_below = XCDR (BVAR (current_buffer, extra_line_spacing));
+          /* Integer pair case.  */
+          if (FIXNATP (line_space_above) && FIXNATP (line_space_below))
+            {
+              int line_space_total = XFIXNAT (line_space_below) + XFIXNAT (line_space_above);
+              it->extra_line_spacing = line_space_total;
+              it->extra_line_spacing_above = XFIXNAT (line_space_above);
+            }
+          /* Float pair case.  */
+          else if (FLOATP (line_space_above) && FLOATP (line_space_below))
+            {
+              double line_space_total = XFLOAT_DATA (line_space_above) + XFLOAT_DATA (line_space_below);
+              it->extra_line_spacing = (line_space_total * FRAME_LINE_HEIGHT (it->f));
+              it->extra_line_spacing_above = (XFLOAT_DATA (line_space_above) * FRAME_LINE_HEIGHT (it->f));
+            }
+          /* Invalid cons. */
+          else
+            {
+              it->extra_line_spacing = 0;
+              it->extra_line_spacing_above = 0;
+            }
+        }
       else if (it->f->extra_line_spacing > 0)
-        it->extra_line_spacing = it->f->extra_line_spacing;
+        {
+          it->extra_line_spacing       = it->f->extra_line_spacing;
+          it->extra_line_spacing_above = it->f->extra_line_spacing_above;
+        }
     }
 
   /* If realized faces have been removed, e.g. because of face
@@ -13157,7 +13194,7 @@ resize_mini_window (struct window *w, bool exact_p)
   else
     {
       struct it it;
-      int unit = FRAME_LINE_HEIGHT (f);
+      int unit;
       int height, max_height;
       struct text_pos start;
       struct buffer *old_current_buffer = NULL;
@@ -13171,6 +13208,10 @@ resize_mini_window (struct window *w, bool exact_p)
 
       init_iterator (&it, w, BEGV, BEGV_BYTE, NULL, DEFAULT_FACE_ID);
 
+      /* Unit includes line spacing if line spacing is added above */
+      unit = FRAME_LINE_HEIGHT (f) +
+          (it.extra_line_spacing_above ? it.extra_line_spacing : 0);
+
       /* Compute the max. number of lines specified by the user.  */
       if (FLOATP (Vmax_mini_window_height))
         max_height = XFLOAT_DATA (Vmax_mini_window_height) * windows_height;
@@ -13203,7 +13244,10 @@ resize_mini_window (struct window *w, bool exact_p)
         }
       else
         height = it.current_y + it.max_ascent + it.max_descent;
-      height -= min (it.extra_line_spacing, it.max_extra_line_spacing);
+
+      /* Remove final line spacing in the mini-window */
+      if (!it.extra_line_spacing_above)
+        height -= min (it.extra_line_spacing, it.max_extra_line_spacing);
 
       /* Compute a suitable window start.  */
       if (height > max_height)
@@ -13241,13 +13285,13 @@ resize_mini_window (struct window *w, bool exact_p)
           /* Let it grow only, until we display an empty message, in which
              case the window shrinks again.  */
           if (height > old_height)
-            grow_mini_window (w, height - old_height);
+            grow_mini_window (w, height - old_height, unit);
           else if (height < old_height && (exact_p || BEGV == ZV))
-            shrink_mini_window (w);
+            shrink_mini_window (w, unit);
         }
       else if (height != old_height)
         /* Always resize to exact size needed.  */
-        grow_mini_window (w, height - old_height);
+        grow_mini_window (w, height - old_height, unit);
 
       if (old_current_buffer)
         set_buffer_internal (old_current_buffer);
@@ -24068,6 +24112,7 @@ append_space_for_newline (struct it *it, bool default_face_p)
             {
               Lisp_Object height, total_height;
               int extra_line_spacing = it->extra_line_spacing;
+              int extra_line_spacing_above = it->extra_line_spacing_above;
               int boff = font->baseline_offset;
 
               if (font->vertical_centering)
@@ -24109,7 +24154,7 @@ append_space_for_newline (struct it *it, bool default_face_p)
 
                   if (!NILP (total_height))
                     spacing = calc_line_height_property (it, total_height, font,
-                                                         boff, false);
+                                                         boff, false);
                   else
                     {
                       spacing = get_it_property (it, Qline_spacing);
@@ -24121,11 +24166,13 @@ append_space_for_newline (struct it *it, bool default_face_p)
                       extra_line_spacing = XFIXNUM (spacing);
                       if (!NILP (total_height))
                         extra_line_spacing -= (it->phys_ascent + it->phys_descent);
+
                     }
                 }
               if (extra_line_spacing > 0)
                 {
-                  it->descent += extra_line_spacing;
+                  it->descent += (extra_line_spacing - extra_line_spacing_above);
+                  it->ascent  += extra_line_spacing_above;
                   if (extra_line_spacing > it->max_extra_line_spacing)
                     it->max_extra_line_spacing = extra_line_spacing;
                 }
@@ -33138,6 +33185,7 @@ void
 gui_produce_glyphs (struct it *it)
 {
   int extra_line_spacing = it->extra_line_spacing;
+  int extra_line_spacing_above = it->extra_line_spacing_above;
 
   it->glyph_not_available_p = false;
 
@@ -33891,7 +33939,8 @@ gui_produce_glyphs (struct it *it)
 
   if (extra_line_spacing > 0)
     {
-      it->descent += extra_line_spacing;
+      it->descent += extra_line_spacing - extra_line_spacing_above;
+      it->ascent  += extra_line_spacing_above;
       if (extra_line_spacing > it->max_extra_line_spacing)
         it->max_extra_line_spacing = extra_line_spacing;
     }