>  > - My left 1920x1080 monitor is
>  > https://www.dropbox.com/s/32f4pijigil3dgf/IMG_6260.JPG?dl=0
>  >
>  > - My right 3840x2160 monitor is
>  > https://www.dropbox.com/s/2advpxq1988uxac/IMG_6259.JPG?dl=0
>  >
>  > - The border between the split windows of Emacs is slightly to the
>  > right of the center of the right monitor.
>
> But this means that your Emacs frame starts before the critical mark
> of 3840 contradicting your earlier response


>
>  >> Does the bug happen only when the
>  >> entire frame is positioned to the right of 3840
>  >
>  > Yes, that's the case.


I hadn't tried split windows at that time.  This time, only the right split
window starts at x > 3840 and exhibits the problem, while the left one does
not, as you can see in the movie.


>
>  > An additional (perhaps important) observation is:
>  >
>  > - The problem occurs only when the right display is added after the X
>  > server (and its first X client - mlterm in my case) started.
>
> Did you start Emacs before adding the right display or after?


The problem occurs in both cases.