Keyboard input for embedded player

I'm using the --parentHWND delayed arguments to embed a Unity player inside a WPF application. Within the application the player is contained in a dockable tab (using the Telerik dockable controls, if it matters). I've got this working almost perfectly with one exception - when I change to a different tab and back again, the Unity player stops receiving keyboard input. I am sending the application an activation message when this happens using the following:

SendMessage(_unityViewerWindowHandle, WM_ACTIVATE, WA_ACTIVE, IntPtr.Zero);

I found that this was necessary to make the mouse input work again after switching tabs, but nothing I do seems to make the keyboard work again. Any help would be greatly appreciated.