Pausing for Javascript to update the html

I'm testing an ExtJS application. The HTML loaded has no content other than a reference to the ExtJS Javascript code. So if I just use <invoke> to load my page its initially blank until the Javascript has a second to update it. Using sleep results in the following message...

"Content of window changed with javascript, it will NOT become current response"

However, that is exactly what I want, I want the javascript change to become the current response. How do I do that??

    <target name="wt.testInWork">
        <webtest name="my simple Test">
            <steps>
                <invoke url="http://localhost:8080/development/commonComponents/superfield/test.html"
                        description="Go to Google (in English)"/>
                <sleep seconds="30"/>
                <verifyText text="SuperField"/> <--- only available after the Javascript has a few seconds to run
            </steps>
        </webtest>
    </target>

-Robert