https://learn.microsoft.com/en-us/dotnet/api/system.windows.contentelement.ismousecapturewithinproperty?view=netframework-4.8.1&viewFallbackFrom=windowsdesktop-10.0
ContentElement.IsMouseCaptureWithinProperty Field (System.Windows) | Microsoft Learn
Identifies the IsMouseCaptureWithin dependency property.
field systemwindowsmicrosoftlearn