TextField size must be explicitly set when customizing the background property

import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.3

Item {
    width: 400
    height: 100
    RowLayout {
        width: 200
        height: 50
        x: 10
        y: 10

        Label {
            text: 'Username:'
        }

        TextField {
            background: Rectangle {
                color: 'red'
            }
        }
    }
}