QQuickView *view = new QQuickView;
view->showFullScreen();

results in a QML window that is fullscreen and which obscures all other windows. e.g.Using Alt+Tab takes the focus away from the application but the window remains on top.

This appears to be related to QTBUG-53515. However the suggested workaround:

QWindowsWindowFunctions::setHasBorderInFullScreen(view, true);

does not work.

EDIT:
For those not familiar with Windows I have attached screenshots of Google Chrome in fullscreen mode and what happens when you then press Alt+tab. Other examples are possible; e.g. when an application is in fullscreen and you press the Windows key, it shows the taskbar and the Windows menu. This doesn't happen for the fullscreen Qt Quick app.