Might also be that Mir expects child windows to be destroyed before their parents. However in X that's not the case (because technically both windows are top-level windows to X; the parenting is introduced in the Xmir layer).
Might also be that Mir expects child windows to be destroyed before their parents. However in X that's not the case (because technically both windows are top-level windows to X; the parenting is introduced in the Xmir layer).