Restore window to correct screen

I have recently added a second monitor to my iMac.

In my NSWindowController I have:

override func windowDidLoad() {     window!.titleVisibility = NSWindow.TitleVisibility.hidden   window!.setFrameAutosaveName(window!.title) }

and in IB, the Restorable attribute for the window is enabled.

This works fine with just one monitor and the window is restored to its previous offsets..

If I move the window to the external monitor, Quit and relaunch the application, the window is restored to the primary monitor at the offsets it had on the external monitor. (i.e. correct position on the screen but incorrect monitor)

Do I need to explicitly save and restore the window screen info or am I missing some other window attribute that should be set.