Question

I have a control in my application that needs to know when any other control in the visual tree captures the mouse.

Is this possible?

Answer 1

Use the Mouse.GotMouseCapture attached event.

public partial class MainWindow : Window {
    public MainWindow() {
        InitializeComponent();
    }
    static MainWindow() {
        EventManager.RegisterClassHandler(typeof(UIElement), Mouse.GotMouseCaptureEvent, new MouseEventHandler(MainWindow_GotMouseCapture));
    }
    static void MainWindow_GotMouseCapture(object sender, MouseEventArgs e) {
        // e.OriginalSource is a captured element
    }
}

Note, that the captured element is available via the Mouse.Captured static property.