Based on CodePlex discussion https://wpftoolkit.codeplex.com/discussions/570377.
"From what I can test, if your custom control is in a LayoutDocument, the floatingWindow will never be empty. But if your Custom control is in a LayoutAnchorable, the floatingWindow won't be empty for the first move, but it will be empty for subsequent moves."
"From what I can test, if your custom control is in a LayoutDocument, the floatingWindow will never be empty. But if your Custom control is in a LayoutAnchorable, the floatingWindow won't be empty for the first move, but it will be empty for subsequent moves."