The result is
I get this result on Windows using Emacs HEAD as well as on Linux using Emacs 30.1 pgtk-build (Arch repo)
While the screenshot has display-line-numbers-mode enabled, it is not necessary to expose the IMHO bug.
To reproduce:
emacs -q --load fc.el fc.el
fc.el attached.
> Alternatively, the value can specify the bitmap directly, with a list
of the form (width height data). Here,
width and height specify the size in pixels, and
data is a string containing the raw bits of the bitmap, row by
row.
so :stipple '(7 1 " ") has a bit mask of "0010 0000" and to my understanding should result in a single black horizontal line. Alas it produces two, which I consider a bug.
This, however, is not consistent. On Linux, when I add more lines to the file so that display-line-numbers-mode would result in a shift of the 'viewport', sometimes only one line is displayed, as expected. On Windows the behaviour remains consistent is so far as always two lines are display