Nav minimap can't drag properly

[linonetwo]

When drag on minimap, the drag delta xy is calculated from the distance from the origin dragstart event point, so it feels like it is drifting. Caused by hammerjs/hammer.js#806

Current workaround #623 (comment)

      const speedupFactorX = this.props.viewrectangle[2] / this.props.width * 2;
      const speedupFactorY = this.props.viewrectangle[3] / this.props.height * 2;

Note that the 2 magic number here is what I test suit for my app size. I thought this.props.viewrectangle[2] / this.props.width is enough, but it is not, don't know why...