The Persona dialog is currently implemented as a popup window. Modern web browsers typically block popups, unless they are triggered directly by a mouse click or a key press. This means that your call to
navigator.id.get() must be made directly from a click or keypress handler. If it's made in response to some other event, the popup won't be shown and the user won't be able to sign in.
For example: suppose your website has deferred sign-in, in which users can browse content, but do not have to sign in until they start editing. If you implement this by calling
navigator.id.request() in response to a focus event on the text field, then the popup will be blocked.
You can still implement sign-in on focus as follows: on the focus event, display a dialog with a "Sign In" button, and have that button call
navigator.id.request() in its click handler.