Since v.2.4.0 the Show(Window owner, ...) does not work properly. The owner is not applied for message box.
Go thru the history of the MessageBox.cs, compare with previous version (commit 106497) and you can find that in method CreateContainer() the line newWindow.Owner = _owner ?? ComputeOwnerWindow(); was removed.
Go thru the history of the MessageBox.cs, compare with previous version (commit 106497) and you can find that in method CreateContainer() the line newWindow.Owner = _owner ?? ComputeOwnerWindow(); was removed.