Reproduction case for PR #266 - scroll & resize listeners not being removed in Internet Explorer 11.

Install with yarn, run with yarn serve.

Once the dev server is running, open http://localhost:8080 (or whatever port the server's running on) in Internet Explorer and make sure you open Developer Tools.

You should see a button labelled Toggle, clicking it should show an (empty) Treeselect.

Now check the Events tab in the Developer Tools, you should see some new event listeners added by vue-treeselect. We're interested in the resize and scroll handlers, which call adjustMenuOpenDirection.

Click on Toggle again and, again, check the Events tab. The resize and scroll listeners should still be there. Moreover, scrolling the page or resizing it should log an unhandled exception:

Unable to get property '$refs' of undefined or null referenceFile: vue-treeselect.js, 
Line: 1479, Column: 7