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

martin