Implicit size of layout

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Hi everybody!

I have a problem with the implicit size of a layout... What I want to do:

I have an application which shows its content in an application window; this content should be able to grow to a certain size; if the window gets bigger, the content should keep a fixed size and only the window should keep growing, showing more and more background.
At the other hand, the window should not be able to shrink to much - it should always be bigger than the minimum size requested by the content. To my understanding all this can be implemented easily by using a layout:

import QtQuick 2.7
import QtQuick.Controls 2.1
import QtQuick.Layouts 1.1

ApplicationWindow {
    id: win
    visible: true

    property bool small: true

    minimumWidth: layout.implicitWidth
    minimumHeight: layout.implicitHeight

    ColumnLayout {
        id: layout
        anchors.fill: parent

        Rectangle {
            id: rect
            Layout.fillWidth: true
            Layout.fillHeight: true
            Layout.alignment: Qt.AlignCenter
            Layout.minimumWidth: small ? 200 : 300
            Layout.minimumHeight: small ? 300 : 400
            Layout.maximumWidth:  500
            Layout.maximumHeight: 600
            color: "black"
        }
    }

    Text {
        anchors.centerIn: parent
        color: "white"
        font.pixelSize: 10
        text:  "rect: " + rect.width + "/" + rect.height + "\n"
             + "rect(min): " + rect.Layout.minimumWidth + "/" + rect.Layout.minimumHeight + "\n\n"
             + "layout(impl): " + layout.implicitWidth + "/" + layout.implicitHeight + "\n\n"
             + "win: " + win.width + "/" + win.height + "\n"
             +  "win(min): " + win.minimumWidth + "/" + win.minimumHeight

    }

    MouseArea {
        id: mouseArea
        anchors.fill: parent
        onClicked: {
            small = !small
        }
    }
}

I use the implicit size of the layout as minimum size of the window (in my real application the content in the layout can be rather difficult, so I do not want to figure out the necessary minimum size by hand...), and a button to change the minimum size requirements of the content.

The above program works as I want it most of the time; however, if I start it, resize the window by dragging one corner with the mouse, and only then begin the clicking, the implicit size of the layout goes to a fixed value, and does not change any more, independent of the minimum size requested by the layout content.
It is not clear to me, why this is the case. Does anybody have an idea how to fix this?

Thanks a lot!!!