changes event.which to event.button to adhere to MDN standards

src/containers/stage.jsx

@@ -179,7 +179,7 @@ class Stage extends React.Component {
         this.updateRect();
         const {x, y} = getEventXY(e);
         const mousePosition = [x - this.rect.left, y - this.rect.top];
-        if (e.which !== 3) {
+        if (e.button === 0) {
             this.setState({
                 mouseDown: true,
                 mouseDownPosition: mousePosition,