I am running MATE 1.20.0-lp150.1.1 desktop (mate-menus 1.20.0-lp150.1.2).
The versionf gdm is 3.26.2.1-lp150.10.1

When accessing the menu, I get the following warning message in syslog:

Jul  1 10:43:30 jeeves /usr/lib/gdm/gdm-x-session[1362] Window manager warning:
Buggy client sent a _NET_ACTIVE_WINDOW message with a timestamp of 0 for
0x2600008 ()
Jul  1 10:43:30 jeeves /usr/lib/gdm/gdm-x-session[1362] Window manager warning:
meta_window_activate called by a pager with a 0 timestamp; the pager needs to
be fixed.

The menu does work correctly, i.e., the message does not seem to impact the
functionality.