When a QML Window has minimumWidth and minimumHeight set to 300 then it will not show its contents until something causes an update. This works fine in Qt Quick Controls 1 ApplicationWindow and seemingly with other sizes too.

import QtQuick 2.8
import QtQuick.Window 2.2

Window {
    minimumWidth: 300
    minimumHeight: 300
    visible: true
    Text {
        text:"This is a test"
    }
}