🚀 go-pugleaf

    A simple example of the kind of problem we have been having is illustrated
    by the memo example on Page 42 of Douglas Young's book on X programming.
    This example works fine under the older versions of X11/Motif. Under the
    new versions we find that the program dumps core if the environment
    variable DISPLAY is not set, and specifying "-display foo:0" does not
    help!

The example calls XtOpenDisplay specifying display_name as the empty
string ("").  This causes XtOpenDisplay to ignore any -display option
in the command line (your symptom #2) and passes the empty string to
Xlib.  The semantics of specifying the empty string to XOpenDisplay
are unspecified, but the usual behavior is to treat this as equivalent
to passing a NULL pointer; i.e. evaluate the DISPLAY envvar.  If
DISPLAY is undefined, XOpenDisplay and XtOpenDisplay return NULL and
the example program does not check for this case, resulting in your
symptom #1.

I've no idea how this could have worked any differently prior to
installing the patches.