A 'screen' XRInputSource should be created and exposed via a session's 'inputsourceschange' event before being found in the input sources. As per the spec:

> Note: Some input sources, like an XRInputSource with targetRayMode set to screen, will only be added to the session’s list of active input sources immediately before the selectstart event, and removed from the session’s list of active input sources immediately after the selectend event.

Currently, tapping the XRPresentationContext's canvas creates the appropriate XRInputSource in `session.getInputSources()` and removes it after no longer touching the canvas. In the case where only a select event is needed (not a controller's pose), listening to a 'inputsourceschange' event followed by an input's 'selectend' event would be sufficient. As a fallback/workaround, polling for input sources on every frame works.

Chrome Version: Chrome Canary 73.0.3632.4
OS: Android