bug#29597: 26.0.90; line-number-display-width returns incorrect width

[Noam Postavsky <npostavs@users.sourceforge.net>]

Paul Rankin <hello@paulwrankin.com> writes:

> I had a look in the source of display-line-numbers.el in the repo but cannot find the definition of line-number-display-width.

Here's the help text:

    line-number-display-width is a built-in function in ‘C source code’.

    (line-number-display-width &optional PIXELWISE)

    Return the width used for displaying line numbers in the selected window.
    If optional argument PIXELWISE is the symbol ‘columns’, return the width
    in units of the frame’s canonical character width.  In this case, the
    value is a float.
    If optional argument PIXELWISE is t or any other non-nil value, return
    the width as an integer number of pixels.
    Otherwise return the value as an integer number of columns of the face
    used to display line numbers, ‘line-number’.  Note that in the latter
    case, the value doesn’t include the 2 columns used for padding the
    numbers on display.