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>

1 files changed, 5 insertions, 2 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,