bug#28879: 26.0.90; Narrowed line numbers shown with display-line-numbers-widen

[Andy Moreton]

From emacs -Q:
1) Open a lisp file
   (setq display-line-numbers-widen t)
   M-x display-line-numbers-mode
2) Move point inside a function
   M-x narrow-to-defun
3) M-x widen

After (2) the window shows line numbers relative to the start of
the buffer, as expected.

After (3), the window shows line numbers starting from 1 for the
narrowed region. After the next mouse or keyboard input, the line
numbers are displayed normally.

Thus it seems that when display-line-numbers-widen is non-nil the line
numbers are not recomputed after widen until then next input.

In GNU Emacs 26.0.90 (build 9, x86_64-w64-mingw32)
 of 2017-10-17 built on QUIETUS
Repository revision: 2e1b3522b8562f94a98fed07aeddb6b5ba0dbc6c