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, 7 insertions, 0 deletions

diff --git a/doc/emacs/display.texi b/doc/emacs/display.texi
index 406feb8c127..cb37ef448e8 100644
--- a/doc/emacs/display.texi
+++ b/doc/emacs/display.texi
@@ -1855,6 +1855,13 @@ the variable @code{display-line-numbers-widen} to a non-@code{nil}
 value, line numbers will disregard any narrowing and will start at the
 first character of the buffer.
 
+@vindex display-line-numbers-offset
+If the value of @code{display-line-numbers-offset} is non-zero, it is
+added to each absolute line number, and lines are counted from the
+beginning of the buffer, as if @code{display-line-numbers-widen} were
+non-@code{nil}.  It has no effect when set to zero, or when line
+numbers are not absolute.
+
 @vindex display-line-numbers-width-start
 @vindex display-line-numbers-grow-only
 @vindex display-line-numbers-width