If Internet Explorer 8 is used the preview page does not use Openlayers2, but still uses OL3.
The pattern to match the user agent doesn't actually match any real IE user agent headers.
- Start the 2.15.x binary package
- Use Chrome to select IE7 and IE8 user agents strings (or download an IE8 test VM from Microsoft)
- Open a layer preview
- The page will serve OpenLayers3
- Use the custom User Agent "MSIE 7." or "MSIE 8."
- The page will serve OpenLayers2