How can you detect when someone clicks away to another control which doesn't steal focus?

Question

We have a custom control which has some 'commit' logic in its LostFocus event which fires if a user moves to a different part of the application.

The problem arises because there are several buttons on toolbars and other areas of the application whose 'Focusable' property is set to False. As such, when they are clicked, their respective actions fire, but our control still hasn't committed its changes yet which causes issues.

Since our UI is modular/dynamic in nature, and because it's just good coding practice, these areas don't know about each other, therefore we can't simply add code in the button handlers to make our control commit. Our control needs to instead realize something outside of itself was clicked.

Our initial attempt was to use a class-level event handler in the GotFocus of our control like so...

EventManager.RegisterClassHandler(
    typeof(FrameworkElement),
    PreviewMouseDownEvent,
    new RoutedEventHandler(MyControl_PreviewMouseDownHandler));

private void MyControl_PreviewMouseDown(object sender, RoutedEventArgs e)
{
    // Exit if the sender isn't the original source.
    // This is because we want the deepest-clicked item, but also only want to process this once
    if(sender != e.OriginalSource)
        return;

    var frameworkElement = (FrameworkElement)sender;

    // If the clicked item is a descendant of ThisControl, do nothing.
    if(frameworkElement.IsDescendantOf(NameOfMyControlInstance))
        return;

    CommitChanges();

    // Unsubscribe class handler here
    ??? Can't do!
}

If our control lost focus, we would simply commit our changes and unsubscribe. Additionally, if the handler fired but OriginalSource wasn't within our control, we would also commit our changes and unsubscribe the handler. The problem is you can't unsubscribe class handlers! Why not I have no idea.

So how can we tell that the user has clicked outside of our control onto something which won't steal the focus?