Hold down ctrl-k. XQuartz sends continuous ctrl-k keystrokes as seen in xev.
Let go of ctrl (keep k pressed). XQuartz should stop sending keystrokes, but instead sends continuous k keystrokes.
In Mac OS terminal, hold down ctrl-k and let go of ctrl - nothing comes out. In XQuartz terminal do the same thing and k's start outputting.
This is behaving correctly. I observe this same behavior with Fedora Core 23.