Further information:

* This bug does does not present itself if I start a session under X. The
problem appears to be Wayland-specific.

* I built 28.1 from scratch with "--with-x-toolkit=no" and the bug
disappears. The menu is in the correct location, although it does not look
the same as it did.

* I built 28.1 from scratch with "--with-x-toolkit=yes" and no other
changes and the bug is back.

Thanks for looking into this!