The menu content element "Section Index" can be used to create a kind of table of contents for an individual page. This is familiar from Wikipedia, for example. At the top of longer pages you will find a list of the headings of a page; if you click on a heading, the browser scrolls to the corresponding position on the page.
We also use this content element here on our website for longer pages, for example here on the page about our TYPO3 template.
The problem
If you use this menu on a hidden page that should be displayed "not in the menu", for example because it is only linked to internally but should not appear in the navigation, then no "section index" is displayed. The section simply remains empty.
The cause
In TYPO3 8 and 9, the output of this menu is generated via the MenuProcessor. This MenuProcessor is used for all types of menus, regardless of whether it is a sitemap, an HMENU or a section index.
By default, pages that are not to be displayed in the menu are not displayed.
In the case of the "Section Index", this behavior unfortunately also affects the display of content elements on hidden pages.
The solution
The behavior of the "Section Index" can be controlled via TypoScript. With the following line in the TypoScript setup, the display of hidden pages is activated and thus the "Section Index" can also be used on hidden pages.
tt_content.menu_section.dataProcessing.10.includeNotInMenu = 1
You can find more information on the various data processors here: https: //docs.typo3.org/m/typo3/reference-typoscript/master/en-us/ContentObjects/Fluidtemplate/Index.html#dataprocessing