Rectangle in Scrollview is not visible - qt

Question

I've created a new project and added a scrollview with a rectangle. When I start the project the rectangle is not displayed but if I add an other control it appears. I'm not sure what I'm doing wrong, I want to see the rectangle always.
import QtQuick 2.7
import QtQuick.Window 2.2
import QtQuick.Controls 1.4
Window {
visible: true
width: 640
height: 480
title: qsTr("Hello World")
id: mainWindow
ScrollView{
anchors.left: parent.left
anchors.top: parent.top
anchors.bottom: parent.bottom
anchors.topMargin: 0
anchors.bottomMargin: 0
width: 289
Rectangle{
anchors.fill: parent
color: "blue"
MouseArea
{
anchors.fill: parent
onClicked: console.log("Click")
}
}
}
}
With this code I get the following window (rectangle is not visible):
However if I add a button to this ScrollView...
ScrollView{
anchors.left: parent.left
anchors.top: parent.top
anchors.bottom: parent.bottom
anchors.topMargin: 0
anchors.bottomMargin: 0
width: 289
Rectangle{
anchors.fill: parent
color: "blue"
MouseArea
{
anchors.fill: parent
onClicked: console.log("Click")
}
}
Button{
text:"Test"
}
}
The rectangle appears:
What's wrong with my first code (without the button)?

Answer 1

The ScrollView has two sizes, the size of the control view in the UI, and the size of the content of the ScrollView.
When you add the Rectangle without the Button, you instruct the Rectangle to fill its parent. The parent in this case is the content of the ScrollView. By default that content is empty, and you've instructed the Rectangle to fill this empty space. Therefore there is nothing displayed.
When you add the Button which has an explicit size, it forces the content of the ScrollView to be non-empty, so now the Rectangle has something to fill, thus you see it.