martin rudalics writes:
>> - In the console output, I see
>>
>>    (emacs:27459): Gtk-CRITICAL **:
>> gtk_distribute_natural_allocation: assertion 'extra_space >= 0'
>> failed
>>
>> - I also see this assertion when I resize the frame with the mouse to a
>>    smaller width than those 78 characters.
>
> Does it also happen with ‘frame-resize-pixelwise’ non-nil?

Yes. But at least I know now what really triggers this problem: GTK
throws this assertion when the menubar is not completely visible. This
is also why running Dired triggers this, because it adds a bunch of
additional menu entries. The frame is then resized so that the menu-bar
fits.

I also have to correct myself: The font doesn't seem to have anything to
do with it; it also happens with Bitmap fonts, it was just by chance
that this configuration had the menu-bar disabled.

I realized that the menu-bar is the reason by putting a breakpoint into
g_log through which the assertion is displayed, and you see that
gtk_menu_bar_size_allocate is actually triggering this. I attached the
full and normal backtrace to this file (xbacktrace is empty).

-David