line("w0") breaks diff mode alignment

[tomtomjhj]

Steps to reproduce

Run vim as follows

vim --clean -S <(tee << 'EOF'
    set lines=20
    call setline(1, range(1,60))
    vnew
    call setline(1, range(1,10) + range(50,60))
    windo diffthis
    norm! G
    exe "norm! 30\<C-y>"
EOF
)

Then, run :echo line('w0', 1001) (1001 is the left window) Note that filler lines are no longer properly aligned.

Expected behaviour

The alignment stays intact.

Version of Vim

9.1.759

Environment

ubuntu 22.04

Logs and stack traces

No response