Comment 6 for bug 495733

It would be greyed out if "Allow SendEvents" (control/left/menu) is
enabled (in turn, that could be a resource-difference between GNOME/KDE).

man xterm

       allowSendEvents (class AllowSendEvents)
               Specifies whether or not synthetic key and button events (gen-
               erated using the X protocol SendEvent request) should be inter-
               preted or discarded. The default is "false" meaning they are
               discarded. Note that allowing such events creates a very large
               security hole, and forcefully disables the allowXXXOps
               resources. The default is "false."