There doesn't seem to be a way to get the RadMediaPlayer to support multiple windows. The expand button will expand the main Silverlight window; not the window the RadMenuPlayer is running in. Here's how I worked around it:
private void MediaPlayer_OnFullScreenChanged(object sender, EventArgs e)
//handle the event
if (sender is RadMediaPlayer)
var mp = (RadMediaPlayer) sender;
mp.IsFullScreen = false;
//Always set to false. Look at a timer.
if (lastFired < DateTime.UtcNow.AddMilliseconds(-100))
if (window.WindowState == WindowState.Normal)
window.WindowState = WindowState.Maximized;
else if (window.WindowState == WindowState.Maximized)
window.WindowState = WindowState.Normal;
lastFired = DateTime.UtcNow;
lastFired is a class variable; as is window.
window = Window.GetWindow(this);