If your website is OnePage only, you don't need Menu from Helix.
Just remove this Menu position from Template Options > Layout
Paul, I have spent over one man-day trying to find a solution to this issue.
The issue, I believe, cannot just be accredited to CSS/HTML.
Joomshaper have several one page templates on Helix 2 and Helix 3 with active item or current item functionality that work.
However there are no Helix Ultimate templates, which I find strange as one page templates have become quite commonplace.
Other Joomla framework designers have modern one page templates available.
One option, obviously, is to keep the current website as WordPress but that really would be a last resort.
I have a Joomshaper developer subscription and would like to use this for this project.
I really hope that there is something that Joomshaper can do to help...
Thanks,
Steve