TOC

There is no option for this within RoboHelp although it might be possible to tweak some of the output files after generating WebHelp. Perhaps someone else can provide specific advice for that method.

Another idea would be to make your own TOC. In other words, don't use the TOC built-in to RH. Just make a topic page and manually create a TOC. Then add a link (as a snippet) to the top of each page linking to your TOC.