Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=54037
test_SendMessage_other_thread(2), which is supposed to show that Windows doesn't use an internal sent message for SetParent, tacks this onto another more complex test, which introduces a race condition (see Comment 2 on the bug). I am not sure if it can be fixed in the existing function, perhaps it can and I just can't keep track of all the moving parts well enough, but I see no need to make it so difficult.