emacs line centering

line-centering.patch

From 60532f6087d0f3cc20197da1790d2f5cec0ecbee Mon Sep 17 00:00:00 2001
From: Jesse Nazario <address@hidden>
Date: Sat, 31 Aug 2019 15:35:34 -0300
Subject: [PATCH] Center lines vertically with line-spacing-vertical-center

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.c | 13 +++++++++++++
 src/buffer.h |  4 ++++
 src/xdisp.c  |  8 +++++++-
 3 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/src/buffer.c b/src/buffer.c
index 62a3d66c8b..1b9f96d61b 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;
@@ -956,6 +961,7 @@ 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;
 }
@@ -5190,6 +5196,7 @@ init_buffer_once (void)
   XSETFASTINT (BVAR (&buffer_local_flags, header_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
@@ -5259,6 +5266,7 @@ 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);
@@ -6248,6 +6256,11 @@ 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 will 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 2080a6f40b..48cf06d06f 100644
--- a/src/buffer.h
+++ b/src/buffer.h
@@ -740,6 +740,10 @@ #define BVAR(buf, field) ((buf)->field ## _)
      in the display of this buffer.  */
   Lisp_Object extra_line_spacing_;
 
+  /* When non-nil will 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 75bc536cb9..0ddabe6e09 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -29294,7 +29294,13 @@ gui_produce_glyphs (struct it *it)
 
   if (extra_line_spacing > 0)
     {
-      it->descent += extra_line_spacing;
+      if (! 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;
     }
-- 
2.23.0

putting here for future reference since gnu seems in no rush to add this (for w/e reason).

To use:

1. curl -L https://gist.githubusercontent.com/jstaursky/375d5b158507c649121b7a1470066e13/raw/0b7624781aa005d2f58c1930cb5ec339efd71f9a/line-centering.patch > <emacs src directory>
2. patch -ruN -d `pwd` < line-centering.patch

BTW credit to https://lists.gnu.org/archive/html/emacs-devel/2019-08/msg00659.html

adds the variable line-spacing-vertical-center