Re: Patch to vertically center line content when using line-spacing variable

[Lars Ingebrigtsen]

email@johnmuhl.me writes:

> I am interested in this landing and contacted (and cc'd) Jesse and the
> FSF paperwork was completed around June 2020. Is there anything else
> holding this back now?
>
> https://lists.gnu.org/archive/html/emacs-devel/2020-01/msg00721.html
> has links to the previous discussion.

I've included the patch below.

I've never used the `line-spacing' variable myself (and my guess is that
few do, which is probably why your message didn't get any responses),
but I did a

  (setq line-spacing 20)

now, and I see that this puts more space at the bottom of the line.  The
proposed patch allows centring instead, which seems eminently reasonable
to me.  (Perhaps this should even be the default?)

Does anybody have any comments here before I apply the patch?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

>From 1302c040a23837b80a53416421914f3fe8ba1926 Mon Sep 17 00:00:00 2001
From: Sungbin Jo <pcr910303@icloud.com>
Date: Fri, 24 Jan 2020 00:56:38 +0900
Subject: [PATCH] Add new variable for centering lines vertically.

Co-authored-by: Jesse Nazario <jessenzr@gmail.com>

When using line-spacing, the new variable line-spacing-vertical-center
can be set to non-nil to center the line content vertically.

* src/buffer.h (struct buffer): New member line_spacing_vertical_center.
* src/buffer.c (bset_line_spacing_vertical_center): New setter for
line_spacing_vertical_center.
(reset_buffer): Reset line_spacing_vertical_center.
(init_buffer_once): Initialize line_spacing_vertical_center.
(syms_of_buffer): Declare Lisp variable line-spacing-vertical-center.
* src/xdisp.c (gui_produce_glyphs): Center the text when
line-spacing-vertical-center is non-nil.
* doc/lispref/display.texi (Line Height): Document the new variable.
* etc/NEWS: Announce the new variable.
---
 doc/lispref/display.texi |  5 +++++
 etc/NEWS                 |  4 ++++
 src/buffer.c             | 14 ++++++++++++++
 src/buffer.h             |  4 ++++
 src/xdisp.c              | 10 +++++++++-
 5 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index e4980fe4c3..2fa63ddb56 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -2231,6 +2231,11 @@ Line Height
 number of pixels put below lines.  A floating-point number specifies
 the spacing relative to the frame's default line height.
 
+  The @code{line-spacing} parameter creates the space by putting some
+margin only below the line.  If you wish to center the line vertically
+instead, you can set the variable @code{line-spacing-vertical-center}
+as non-nil.
+
 @vindex line-spacing
   You can specify the line spacing for all lines in a buffer via the
 buffer-local @code{line-spacing} variable.  An integer specifies
diff --git a/etc/NEWS b/etc/NEWS
index 11ef31b2c8..fb85131f07 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -51,6 +51,10 @@ It was declared obsolete in Emacs 27.1.
 
 * Changes in Emacs 28.1
 
+** New variable line-spacing-vertical-center.
+This variable, if non-nil, centers the line content vertically when
+using the 'line-spacing' variable.
+
 
 * Editing Changes in Emacs 28.1
 
diff --git a/src/buffer.c b/src/buffer.c
index 5c65d4d4d1..234745a66a 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -219,6 +219,11 @@ bset_extra_line_spacing (struct buffer *b, Lisp_Object val)
   b->extra_line_spacing_ = val;
 }
 static void
+bset_line_spacing_vertical_center (struct buffer *b, Lisp_Object val)
+{
+  b->line_spacing_vertical_center_ = val;
+}
+static void
 bset_file_format (struct buffer *b, Lisp_Object val)
 {
   b->file_format_ = val;
@@ -967,6 +972,8 @@ reset_buffer (register struct buffer *b)
     (b, BVAR (&buffer_defaults, enable_multibyte_characters));
   bset_cursor_type (b, BVAR (&buffer_defaults, cursor_type));
   bset_extra_line_spacing (b, BVAR (&buffer_defaults, extra_line_spacing));
+  bset_line_spacing_vertical_center (b, BVAR (&buffer_defaults,
+                                              line_spacing_vertical_center));
 
   b->display_error_modiff = 0;
 }
@@ -5202,6 +5209,7 @@ init_buffer_once (void)
   XSETFASTINT (BVAR (&buffer_local_flags, tab_line_format), idx); ++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, cursor_type), idx); ++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, extra_line_spacing), idx); ++idx;
+  XSETFASTINT (BVAR (&buffer_local_flags, line_spacing_vertical_center), idx); 
++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, cursor_in_non_selected_windows), 
idx); ++idx;
 
   /* buffer_local_flags contains no pointers, so it's safe to treat it
@@ -5272,6 +5280,7 @@ init_buffer_once (void)
   bset_bidi_paragraph_separate_re (&buffer_defaults, Qnil);
   bset_cursor_type (&buffer_defaults, Qt);
   bset_extra_line_spacing (&buffer_defaults, Qnil);
+  bset_line_spacing_vertical_center (&buffer_defaults, Qnil);
   bset_cursor_in_non_selected_windows (&buffer_defaults, Qt);
 
   bset_enable_multibyte_characters (&buffer_defaults, Qt);
@@ -6268,6 +6277,11 @@ from (abs POSITION).  If POSITION is positive, point was 
at the front
 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.  
*/);
 
+  DEFVAR_PER_BUFFER ("line-spacing-vertical-center",
+                    &BVAR (current_buffer, line_spacing_vertical_center), Qnil,
+                     doc: /* Non-nil means center the line content vertically
+when using `line-spacing' variable.  */);
+
   DEFVAR_PER_BUFFER ("cursor-in-non-selected-windows",
                     &BVAR (current_buffer, cursor_in_non_selected_windows), 
Qnil,
                     doc: /* Non-nil means show a cursor in non-selected 
windows.
diff --git a/src/buffer.h b/src/buffer.h
index fd05fdd37d..c15065b599 100644
--- a/src/buffer.h
+++ b/src/buffer.h
@@ -553,6 +553,10 @@ #define BVAR(buf, field) ((buf)->field ## _)
      in the display of this buffer.  */
   Lisp_Object extra_line_spacing_;
 
+  /* Non-nil means center the line content vertically. To be used
+     along with `line-spacing'.  */
+  Lisp_Object line_spacing_vertical_center_;
+
   /* Cursor type to display in non-selected windows.
      t means to use hollow box cursor.
      See `cursor-type' for other values.  */
diff --git a/src/xdisp.c b/src/xdisp.c
index a5efbb39be..44727cdd64 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -30527,7 +30527,15 @@ gui_produce_glyphs (struct it *it)
 
   if (extra_line_spacing > 0)
     {
-      it->descent += extra_line_spacing;
+      if (NILP (BVAR (XBUFFER (it->w->contents),
+                      line_spacing_vertical_center)))
+        it->descent += extra_line_spacing;
+      else
+        {
+          int spacing = extra_line_spacing / 2;
+          it->ascent += spacing;
+          it->descent += spacing;
+        }
       if (extra_line_spacing > it->max_extra_line_spacing)
        it->max_extra_line_spacing = extra_line_spacing;
     }
-- 
2.21.1 (Apple Git-122.3)