Whoops - my bad. Apparently in the distant past I had set my mouse to keystroke Ctrl-N when pushing the centre button, so yeah, X1 does conform to the norm and I'm the odd duck. And upon further investigation, I hadn't previously noticed that one can set program-specific actions for mouse buttons in Windows. Convenient, that.

Now if I can just figure a way to get the scroll wheel to zoom in and out in X1 in nice, smallish increments, my life will be complete...