> The answer, which I sort of suspected, is that it never hit that
 > breakpoint at all before the assertion fires.

You're right.  In

+		if (!NILP (Fstring_equal (XBUFFER (w->contents)->name_,
+					  build_string ("*Server*"))))
+		  best_window = Qt;
+
  		/* Undedicate WINDOW.  */
  		wset_dedicated (w, Qnil);
  		/* Make WINDOW show the buffer returned by
@@ -3048,6 +3061,8 @@ window_loop (enum window_loop type, Lisp_Object obj, bool mini,
  		if (EQ (window, selected_window)
  		    && XBUFFER (w->contents) == current_buffer)
  		  Fset_buffer (w->contents);
+
+		replace_buffer_in_windows_safely_count += 1;

replace_buffer_in_windows_safely_count is incremented regardless of
whether W's buffer is *Server* or not.  In either case the assertion
would not fail.  So once we are here everything is all right and the bug
must be that we do not get here.  I attach the next patch.  Please put a
breakpoint at the

       replace_buffer_in_windows_safely_count = 0;

and try to step through the rest of this function with GDB.  And if you
don't hit it, the detection logic went wrong ...

martin