OK. In the next step I'd like to isolate the menubar code as the sole
culprit for what's going in. Please with master do
(setq default-frame-alist '((width . 200)))
It takes effect on the initial frame, but doesn't affect the size of the frame opened with C-x 5 2, or remove the gtk error message:
(emacs:2091071): Gtk-CRITICAL **: 15:57:43.714: gtk_distribute_natural_allocation: assertion 'extra_space >= 0' failed
(Just to confirm: I put this setting in early-init.el.)
I then applied the trivial patch you gave to git master HEAD, ran 'emacs -Q' and did C-x 5 2, and got a small window as usual, but with no error message in the terminal.