On mouse move callback
function onMouseMove(event) {
      if (ctrlDown) {
        if (drag.start.x === null) {
          drag.start.x = event.clientX;
          drag.start.y = event.clientY;
        }
        let threshold = 15;

        stackHelper.slice.intensityAuto = false;

        let dynamicRange = stack.minMax[1] - stack.minMax[0];
        dynamicRange /= threeD.clientWidth;

        if (Math.abs(event.clientX - drag.start.x) > threshold) {
          // window width
          stackHelper.slice.windowWidth += dynamicRange * (event.clientX - drag.start.x);
          drag.start.x = event.clientX;
        }

        if (Math.abs(event.clientY - drag.start.y) > threshold) {
          // window center
          stackHelper.slice.windowCenter -= dynamicRange * (event.clientY - drag.start.y);
          drag.start.y = event.clientY;
        }
      }
    }