bug#39431: window-text-pixel-size behavior when enabled display-line-number-mode

[tsuucat]

Hi,
I found unexpected behavior (for me) of window-text-pixel-size.

Environment:
GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30, 
cairo version 1.15.10)

Steps:
1. run emacs -Q.
2. In *scratch* buffer eval following.
(window-text-pixel-size nil (point-min) (1+ (point-min)))
;=> (9 . 18)
(display-line-numbers-mode)
;=> t
(window-text-pixel-size nil (point-min) (1+ (point-min)))
;=> (45 . 18) ; width changed

I also found this behavior in NS port Emacs 27.0.50.

Is this expected?

--
tsuucat