>>
>> gtk_distribute_natural_allocation: assertion 'extra_space >= 0' failed
>>
>
> Yes, both with Emacs 29 and git master produce this message when
> menu-bar-mode is non-nil, and the menu bar is drawn, in both window sizes
> (the normal sized window, and the strangely small one).
But you can't see this message when building with gtk-2 I presume.
That's correct, no message with gtk-2.
This strongly hints that there was an Emacs change affecting gtk-2 _and_
gtk-3 builds in between 29 and master. I could imagine that commit
e087c89b1e243bbd941a4a50b4bf99613e13d016
is involved but if you could try to bisect, it would be of great help.
Just to check, you're suggesting that I bisect from emacs-29.3 (I use that tag because it's what the version of 29 I'm using is built from) to master HEAD, to find at which commit C-x 5 2 opens a small window immediately (without disabling menu-bar-mode)?
BTW, could you try adding a (user-size . t) member to 'default-frame-alist'?
And could you try doing that from an 'early-init.el' file?
This made no difference. I added just:
(add-to-list 'default-frame-alist '(user-size . t))
to ~/.config/emacs/early-init.el
Then I ran each Emacs with just "emacs", confirmed that default-frame-alist was set, then disabled menu-bar-mode (Emacs 29 only), then C-x 5 2. In each case a small window was opened as before.