There is a “black screen” problem in Netbeans 7.4 embedded WebKit browser, this seems to be related to Bug 236961. The problem is caused by a canvas added by Netbeans at the end of the previewed page. My workaround is simple, I’m adding the following code to my jQuery enabled application I’m currently working on:

<script>
    $(document).ready(function(){
        function hideNetbeansGlassPane() {
            $('#netbeans_glasspane').remove();
        }
        setTimeout(hideNetbeansGlassPane, 500);
    });
</script>

And the black netbeans_glasspane is gone ;-)

Not so clean, but problem is solved.