Fix issue #469 (#523)

This looks good to me!  My only comment is that the title of the PR should be descriptive, with the comment exactly as you have it.  I believe the issue should close automatically if the comment is correctly phrased. We'll  see!

Author: ben-lu-uw

src/mapml/handlers/ContextMenu.js

@@ -433,7 +433,7 @@ export var ContextMenu = L.Handler.extend({
       this._layerClicked = undefined;
       this._showAtPoint(e.containerPoint, e, this._container);
     }
-    if(e.originalEvent.button === 0){
+    if(e.originalEvent.button === 0 || e.originalEvent.button === -1){
       this._keyboardEvent = true;
       this._container.firstChild.focus();
     }