How to drag an Openseadragon canvas with mouse middle wheel button

I am using Openseadragon library with fabricjs overlay. I have a case where I want to drag the canvas but instead of mouse primary button, I want to drag it with middle mouse button press. Could anyone please help me get the desired behavior?

OpenSeadragon doesn't have a flag for that, but you can easily build it using the MouseTracker. Here's an example (coded from memory and not tested, but it should give you the idea).

var drag;

var mouseTracker = new OpenSeadragon.MouseTracker({
  element: viewer.container, 
  nonPrimaryPressHandler: function(event) {
    if (event.button === 1) { // Middle
      drag = { 
        lastPos: event.position.clone()
      };
    }
  },
  moveHandler: function(event) {
    if (drag) {
      var deltaPixels = drag.lastPos.minus(event.position);
      var deltaPoints = viewer.viewport.deltaPointsFromPixels(deltaPixels);
      viewer.viewport.panBy(deltaPoints);
      drag.lastPos = event.position.clone();
    }
  },
  nonPrimaryReleaseHandler: function(event) {
    if (event.button === 1) {
      drag = null;
    }
  }
});

EDIT: I had a bug in the example code above; fixed.