Why root level pages cannot be expanded?

[neo-mashiro]

nav:
  - Home: index.md
  - Getting started:
    - Installation: getting-started.md
    - Creating your site: creating-your-site.md
    ......
    - License: license.md
    - Tutorials:
      - tutorials/index.md
      - "Blogs":
        - tutorials/blogs/basic.md
        - tutorials/blogs/navigation.md
        - tutorials/blogs/engage.md
      - "Social cards":
        - tutorials/social/basic.md
        - tutorials/social/custom.md

Hello, I have a simple question. Why are root level pages not expandable? Is this by design?

Here I'm using the Getting Started tab as an example. Within this tab, we have a couple root level pages like Creating your site, Customization. From both YAML and the screenshot, you can see that Tutorials is also a root level page (at level 1), which has two child pages Blogs and Social cards (at level 2), the Blogs page then has its own child pages like Basic blogs (at level 3).

While level 2 pages have a "expand/collapse" mark on the right to show/hide its child pages at level 3, root level pages don't have such a mark. Instead, level 2 pages are always expanded just like root level pages, there's no way to hide them and they even have the same level of indentation as root pages. I wonder why it behaves like this.

In my case, I have a root level page that contains a bunch of child pages at level 2, I just want to be able to show/hide them using a "expand/collapse" mark on the right, but that doesn't seem to be supported.

[neo-mashiro]

....
    - License: license.md                  # level 1 (root)
    - Tutorials:                           # level 1 (root)
      - tutorials/index.md
      - "":                                # level 2 <- make a dummy section here
        - "Blogs":                         # level 3 <- move the original level 2 branch one level deeper
          - tutorials/blogs/basic.md       # level 4
          - tutorials/blogs/navigation.md  # level 4
          - tutorials/blogs/engage.md      # level 4

Finally I found a workaround. Knowing that level 2 pages behave the same as root level pages, I can just make a dummy empty section - "": at level 2 and move the original level 2 branch underneath this level (so that they are now one level deeper). Unfortunately, the empty section - "": also introduces some vertical spacing that doesn't look good.

It seems to me that this is a limitation of mkdocs itself which just got carried over to mkdocs-material.

[neo-mashiro]

OK I figured out the solution. Once I turned off the navigation.sections feature everything works fine.

theme:
  features:
    - navigation.sections

The documentation clearly states that

When sections are enabled, top-level sections are rendered as groups in the sidebar...
If both feature flags navigation.tabs and navigation.sections are enabled, sections are rendered for level 2 navigation items.

So this is expected behavior. Sorry for missing that info when reading the documentation.