How to change Rectangle height that is inside a ColumnLayout?

I've this code:

Code:

---

import QtQuick 2.5
import QtQuick.Layouts 1.1
import QtQuick.Controls 1.4
ColumnLayout {
    spacing: 0
    Rectangle {
        Layout.fillWidth: true
        height: 30
        color: "blue"
        Button {
            text: "expand"
            onClicked: r.height = 50
        }
    }
    Rectangle {
        id: r
        Layout.fillWidth: true
        height: 20
        color: "red"
    }
    Rectangle {
        Layout.fillWidth: true
        Layout.fillHeight: true
        color: "lightgray"
    }
}

---

When the button is pushed, the Rectangle r doesn't change the height. However, if the last Rectangle is changed by an Item, the height is changed.

Interesting.
The height changes but the layout doesn't update.

But it works if you use "implicitHeight" instead of "height" for the red rectangle

Cheers,
_