Stop event propagation

[rreusser]

I end up typing this in nearly every example, otherwise the events leak through and, for example, pan the underlying view even when I'm dragging a slider:

var el = document.querySelector('.control-panel');
var events = ['mousemove', 'click', 'mousedown', 'mouseup'];
for (var i = 0; i < events.length; i++) {
  el.addEventListener(events[i], function(ev) {
    ev.stopPropagation();
  });
}

Note to self: make a PR for this! I wonder what the full set of events stopped from propagating should be.