When an element captures the mouse, it receives mouse input whether or not the cursor is within its borders. The mouse is typically captured in this manner only during drag-and-drop operations, and retains capture until the drop action of the drag-and-drop operation occurs.
// Mouse.Capture returns an IInputElement.
captureElement = Mouse.Captured;
// Update the Label that displays the element with mouse capture.
lblHasMouseCapture.Content = captureElement.ToString();