Weston crashes because shell is trying to set a transient surface without having a parent. The real problem though is in XWM which always assign transient shell type for X override-redirected windows, in particular the ones that don't have a parent. To fix for this behaviour, we have to introduce a special shell type for X windows, where X windows are positioned globally and not depending on their parents for anything.

To reproduce: start Weston with Xwayland support, start gedit then "File" -> "Open" and press ESC. Weston segfaults with the following backtrace:

Program received signal SIGSEGV, Segmentation fault.
0x00007fffefae22d8 in set_surface_type (shsurf=0x9f1a80) at shell.c:1496
1496					pes->geometry.y + shsurf->transient.y);
(gdb) bt
#0  0x00007fffefae22d8 in set_surface_type (shsurf=0x9f1a80) at shell.c:1496
#1  0x00007fffefae5aee in shell_surface_configure (es=0x9f0490, sx=0, sy=0)
    at shell.c:3039
#2  0x000000000040a757 in surface_commit (client=0x9a9d10, resource=0x9f0490)
    at compositor.c:1401
#3  0x00007ffff635ea14 in ffi_call_unix64 ()
   from /usr/lib/x86_64-linux-gnu/libffi.so.6
#4  0x00007ffff635e435 in ffi_call ()
   from /usr/lib/x86_64-linux-gnu/libffi.so.6
#5  0x00007ffff7bd3404 in wl_closure_invoke (closure=0x9efc70, 
    target=0x9f0490, func=0x40a65a <surface_commit>, data=0x9a9d10)
    at connection.c:908
#6  0x00007ffff7bcc63b in wl_client_connection_data (fd=32, mask=1, 
    data=0x9a9d10) at wayland-server.c:297
#7  0x00007ffff7bd06a8 in wl_event_source_fd_dispatch (source=0x962590, 
    ep=0x7fffffffd7a0) at event-loop.c:84
#8  0x00007ffff7bd1076 in wl_event_loop_dispatch (loop=0x6279b0, timeout=-1)
    at event-loop.c:415
#9  0x00007ffff7bce4a3 in wl_display_run (display=0x627950)
    at wayland-server.c:1207
#10 0x000000000040f12d in main (argc=1, argv=0x7fffffffdc88)
    at compositor.c:3356
(gdb)