Implement offsets for absolute line numbers

* src/xdisp.c (syms_of_xdisp) <display-line-numbers-offset>:
New variable to add an offset to absolute line numbers.
(syms_of_xdisp) <display-line-numbers>: Mention it in docstring.
(maybe_produce_line_number): Use it.

* doc/emacs/display.texi (Display Custom): Document it.

* etc/NEWS (value): Announce it.

* lisp/frame.el: Add `display-line-numbers-offset' to list of
variables which should trigger redisplay of the current buffer.

1 files changed, 4 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 2ca681ff9b9..49aa7f6007b 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -560,11 +560,15 @@ now prompts the user for the directory containing the desktop file.
 
 +++
 ** display-line-numbers-mode
+
 *** New faces 'line-number-major-tick' and 'line-number-minor-tick',
 and customizable variables 'display-line-numbers-major-tick' and
 'display-line-numbers-minor-tick' can be used to highlight the line
 numbers of lines multiple of certain numbers.
 
+*** New variable `display-line-numbers-offset', when non-zero, adds
+an offset to absolute line numbers.
+
 +++
 ** winner
 *** A new variable, 'winner-boring-buffers-regexp', has been added.