Where do margins come from in this QML sample?
<p>I've looked at tons of questions on SO about content margins in QML but all of the answers point to missing <code>spacing: 0</code> properties. I've done all that, but still get weird spaces that I cannot eliminate. Can anyone explain why this QML code:</p>

<pre><code>import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.0

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Test")

    RowLayout {
        spacing: 0
        anchors.margins: 0, 0, 0, 0
        anchors.fill: parent;

        Pane {
            anchors.margins: 0, 0, 0, 0
            id: menuPane
            anchors.top: parent.top;
            anchors.bottom: parent.bottom;
            width: 200

            ColumnLayout {
                spacing: 0
                anchors.fill: parent
                anchors.margins: 0, 0, 0, 0

                Rectangle {
                    id: testRect
                    Layout.fillWidth: true
                    anchors.top: parent.top
                    anchors.left: parent.left
                    anchors.right: parent.right
                    height: 20
                    color: "green"
                }
            }
        }

        Pane {
            anchors.margins: 0, 0, 0, 0
            anchors.left: menuPane.right
            anchors.right: parent.right
            anchors.top: parent.top
            anchors.bottom: parent.bottom

            Rectangle {
                anchors.margins: 0, 0, 0, 0
                anchors.fill: parent
                color: "black"
            }
        }
    }
}
</code></pre>

<p>is rendered like this? Why are the margins between the rectangles there?
<a href="https://i.stack.imgur.com/ptGZK.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ptGZK.png" alt="QML window screenshot"></a></p>