When a BACKBASE application is loading, before it gets initialized, it briefly shows the underlying source code to the user. To make sure that this doesn’t happen, use a small trick on the xmp tag:
<xmp b:backbase="true" style="display:none;">
<s:event b:on="construct" b:action="show" />
...
</xmp>
The whole backbase source is hidden with display:none until it is completely loaded and parsed. When the construct event is fired, it’s simply shown again.