$ emacs -Q /usr/share/dict/words
C-SPC
M->
^X^X  .....
(beep) Timed out waiting for property-notify event

You can recover by hitting ^G several times.

Problem appears when the transient-mark encloses a large region.  Does not
occur in a terminal window (i.e., emacs -Q -nw).  Appears to be an
x-selection issue.