Currently it's very hard to implement a popup window that gets automatically dismissed when someone click outside it.
There're some workarounds, like putting a MouseArea under the MainWindow and listen for clicks. However these solutions are very error prone because often they depends on every mousearea to propagate clicked events.
In the private source code there's already a QQuickPopupWindow that implements this behaviour. The only thing that's missing it just exposing it aa creatable Qml.