[openbox] mousebound left release script fires on press, not waiting for release

[Plmalternate Plmalternate]

context:
Openbox 3.6.1
running plain under xorg 1:7.7+13ubuntu3
started with startx from xinit 1.3.4-3ubuntu0.1
No DE per se, no DM, in a minimalist Ubuntu 16.04, 64 bit.

I have this in in the mouse bindings section of ~/.config/openbox/rc.xml:

    <context name="Client">
      <mousebind button="Left" action="Release">
        <action name="Execute">
					<command>check_xclip.sh</command>
				</action>
      </mousebind>
   </context>

but it runs when I press, repeat PRESS, the left button, and doesn't
wait for me to release it.

Duckduckgo couldn't find a bug like this. Is it a bug? Anything I can
do to help figure it out? Failing that, to work around it?