Thanks for the pointer! That is a long thread so I only skimmed it. I
think you meant this snippet in [0]:
(defun project-other-window-command ()
(interactive)
(other-window-prefix)
(set-transient-map (make-composed-keymap project-prefix-map
project-other-window-map)))
Exactly. And the same for the frame command in the most recent patch
I posted to bug#63648, and I confirm that your bug report is fixed with it.
That indeed fixes the issue. My only concern is that
`other-window-prefix' is a new function introduced in Emacs 28.
Indeed, it would take more efforts to provide backward-compatibility
for older Emacs versions.