martin rudalics <rudalics@gmx.at> writes:

>>> in xterm.c please remove the two occurrences of
>>>
>>> #if defined USE_GTK && defined HAVE_GTK3
>>> 	      /* If GTK3 wants to impose some old size here (Bug#24526),
>>> 		 tell it that the current size is what we want.  */
>>> 	      xg_frame_set_char_size
>>> 		(f, FRAME_PIXEL_WIDTH (f), FRAME_PIXEL_HEIGHT (f));
>>> #endif
>>>
>>> and tell me whether the problem persists.
>>
>> Nope, then the problem is gone and also toggling the toolbar and
>> menubar has immediate effect without need for resizing the frame.
>
> Please tell me whether it's necessary to remove both of these to make
> the problem go away.  If removing the second one suffices,

It does suffice.

> please try the attached patch and tell me whether it helps.

When I apply the patch to an unmodified xterm.c, emacs -Q has a menubar
but no toolbar, and the complete UI except for the menubar is just black
screen with no text, no modeline, no nothing!  Even resizing or typing
won't make anything appear.  The menubar works, though.  I can expand it
and press entries, and it seems it reacts to that.

When I apply your patch and also remove the second ifdef, that doesn't
change.  Just a menubar with blackness on the rest of the frame.

Screenshot: