WebStageView and Keeping Focus

I have a desktop app that works fine employing a WebStageView to allow folks to log in using oAuth at a website. One person (out of maybe 2000 clients) recently bought a new Microsoft Surface. When he clicks in the username textinput the caret blinks confirming it has the focus. As soon as he tries to type (using an external keyboard - though I've also temporarily stopped the touch screen keyboard service to  rule it out) the WebStageView loses focus and nothing types into the textInput.

Any ideas/suggestions?