XamDataGrid ActiveDataItem changing on window focus

This is really just built-in behavior/functionality of the WPF framework. When a WPF window loses keyboard focus the internal HwndKeyboardInputProvider stores a reference to the element that had the keyboard focus. Then when the associated window gets focus (e.g. when you activate the window) it automatically focuses that element. That is what is happening here. When you clicked on the record presenter at the top you gave that keyboard focus. When you clicked on another app you took keyboard focus away from the WPF window and so it stored a reference to that RecordPresenter. When you clicked on the window to activate it it focused that cached element - the rp on top. This isn't something that the DataPresenter has any knowledge about or control over so its not something we could change within the control; it is just how the WPF framework is implemented.

In terms of working around this I suppose you could either try to use reflection to manipulate the HwndKeyboardInputProvider or you could try to intercept when WPF is trying to refocus that element. You'll have to verify this in your app and possibly have to make adjustments based on what you find but something like the following might get you started:

static MainWindow()
{
    EventManager.RegisterClassHandler(typeof(RecordPresenter), Keyboard.PreviewGotKeyboardFocusEvent, new KeyboardFocusChangedEventHandler((o, e) =>
    {
        // assuming keyboard focus wasn't in the wpf window (note this could mean it was within an hwndhost, etc.)
        if (e.OldFocus == null)
        {
            var newFocus = e.NewFocus as DependencyObject;
            var focusScope = FocusManager.GetFocusScope(newFocus);
            var focusedElement = FocusManager.GetFocusedElement(focusScope) as DependencyObject;

            // if the element being focused isn't the currently logically focused element of the focus scope and its 
            // not a descendant then we could cancel the focus change
            if (focusedElement != newFocus && !Infragistics.Windows.Utilities.IsDescendantOf(newFocus, focusedElement))
            {
                e.Handled = true;
            }
        }

    }));
}