Topic title appears in two menu levels

[Phlandrew]

I'm sure there's something basic to sort this, but d*mned if I can find it...

Given this TOC

TOCTOC.jpg

In PDF I get this (which I want)

PDFTOC.jpg

But when I do it in HTML5 the parent header is duplicated at the same level as the children:

5TOC.jpg

Clicking on the top heading does nothing, but the other does open the right page... although manifestly it has subtopics.

I'm lost.

[NorthEast]

Clicking on the top heading does nothing, but the other does open the right page... although manifestly it has subtopics.

Clicking the top heading (with the icon) shows/hides the topics inside that book.

So it means you can browse the tree without having to open the topics.
If you don't like how it works, the skin Slide-out menu style can be set to drilldown instead of tree.

[Zerozeddy]

I understand that, but if I click on "Graphical views..." the first thing in the navigation it comes to is "Graphical views". The hierarchy is breaking down. It puts that topic at the same level as "About the dashboard" as well, which is wrong.

What I want/expect is that item in the red box in pic 3 not to be there. It's already there, one step up.