Steps to reproduce

1. Runvim --clean -S test.vim, with

vim9script

set nosplitscroll

setline(1, range(100)->mapnew((_, i: number): string => i .. repeat(' test', 10)))
normal! G
normal! zz
normal! $

def Test()
    const winid: number = popup_atcursor('// test test test', {})
    setbufvar(winbufnr(winid), '&syntax', 'c')
enddef

nnoremap g<cr> <scriptcmd>Test()<cr>

2. While the cursor is located on the last line and centered on the screen, press g<cr>.
3. Result: The window scrolls unexpectedly. In some situations the cursor jumps to the beginning of a line (which I haven't been able to reproduce with a minimal example yet).

The scrolling is definitively caused when setting the syntax for the buffer displayed in the popup window:

setbufvar(winbufnr(winid), '&syntax', 'c')

No issues arise when splitscroll is enabled.

Expected behaviour

No scrolling and no cursor jumps when the syntax is set for a buffer displayed in a popup window.

Version of Vim

9.0.478