Since Windows Phone 8.1, some devices will have show the windows key, back and search keys on screen as soft keys instead of physical buttons. You can read more here. This causes a problem in Cordova/Phone Gap applications and the soft keys will overlap with the web view.
This plugin aims to sort that by automatically resizing the web view as the softkeys are shown/hidden by user action. If you need to, you can also capture the height of the soft key/navbar with the following:
window.addEventListener("navbarchange", function(info)
{
alert("the nav bar is " + info.height + " high!");
}, false);
Whilst this plugin uses properties that are only available on Windows Phone 8.1, it is backwards compatible and will run on earlier version but will do nothing.