Android : Stay in Immersive Full-Screen Mode

As soon as a text field in the WebView receives focus, the on-screen keyboard appears in combination with the ActionBar. This seems to be a bug, IMHO. How can I avoid this?

The relevant parts:

a class that inherits from ActionBarActivity

...
public void hideSystemBar()
{
    getWindow().getDecorView().setSystemUiVisibility(
            View.SYSTEM_UI_FLAG_LAYOUT_STABLE
                    | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
                    | View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
                    | View.SYSTEM_UI_FLAG_HIDE_NAVIGATION
                    | View.SYSTEM_UI_FLAG_FULLSCREEN
                    | View.SYSTEM_UI_FLAG_IMMERSIVE);
}

AndroidManifest.xml

<activity
...
    android:configChanges="keyboard|keyboardHidden|orientation|screenSize"