While playing around with a recent build of dev (82bdc55bc36f8a785e2637b01cb91d361b01fc29), I've noticed that toplevel window transparency for QQuickWindow seems to have stopped working (at least on macOS, haven't tried Windows so far).
import QtQuick 2.7
import QtQuick.Window 2.0
import QtQuick.Controls 1.1
x: (Screen.width - width) / 2
y: (Screen.height - height) / 2
Window is transparent with 5.9 but opaque with dev.
Setting QQuickWindow::setDefaultAlphaBuffer(true) before creating any window does not help and is not necessary on 5.9 anyway.