This code works in the views example, but if you write it in an external editor and load in the browser the button appears at x=0, until the window is resized when it then jumps to the correct position.

    <canvas height="300" width="500">
              <window x="20" y="20" width="200" height="250"
                title="Boring Window" resizable="true">
            <simplelayout axis="y" spacing="10" />
            <text>Here is some text.</text>
            <text>I could ramble for hours.</text>
            <button x="${(immediateparent.width / 2)
                             - (this.width / 2)}">
                My Button
            </button>
        </window>
    </canvas>