Hi Paul, FYI, The fix had been included the v1.6.7 release of app-bridge.  

Hi Paul, First of all thank you for providing all the details of your current project setup. This actually help us discover an obscure bug that took some time to track down. We are in the process of fixing with it with app-bridge, and will update her...

Try running your app inside Shopify admin. App Bridge is the connection between your app and Shopify admin and doesn't really work without an Admin Frame.