TOC in sidebar no matter what for index.qmd

I have:

• searched the issue tracker for similar issues
• installed the latest version of Quarto CLI
• formatted my issue following the Bug Reports guide

Bug description

The TOC on the right shows up for the index.qmd file, even though it shouldn't.

Steps to reproduce

1. Create the default Quarto book project in Rstudio.
2. Render the HTML and view the site

Actual behavior

There is a TOC section on the right:

Expected behavior

It should look like the other three pages in the book, without a TOC on the right:

Your environment

• IDE: Rstudio Version 2025.09.0+387 (2025.09.0+387)
• OS: Mac OS 26

Quarto check output

Quarto 1.8.24
[✓] Checking environment information...
Quarto cache location: /Users/jtbayly/Library/Caches/quarto
[✓] Checking versions of quarto binary dependencies...
Pandoc version 3.6.3: OK
Dart Sass version 1.87.0: OK
Deno version 2.3.1: OK
Typst version 0.13.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
Version: 1.8.24
Path: /Applications/quarto/bin

[✓] Checking tools....................OK
TinyTeX: (not installed)
Chromium: (not installed)

[✓] Checking LaTeX....................OK
Using: Installation From Path
Path: /Library/TeX/texbin
Version: 2020

[✓] Checking Chrome Headless....................OK
Using: Chrome found on system
Path: /Applications/Google Chrome.app/Contents/MacOS/Google Chrome
Source: MacOS known location

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
Version: 3.13.5
Path: /usr/local/opt/python@3.13/bin/python3.13
Jupyter: (None)

  Jupyter is not available in this Python installation.
  Install with python3 -m pip install jupyter

[✓] Checking R installation...........OK
Version: 4.3.0
Path: /Library/Frameworks/R.framework/Resources
LibPaths:
- /Library/Frameworks/R.framework/Versions/4.3-arm64/Resources/library
knitr: 1.43
rmarkdown: 2.24

[✓] Checking Knitr engine render......OK

Start by creating the default Quarto book project in RStudio and rendering its HTML, then compare index.qmd with the other three pages. Reproduce the unwanted right-side TOC and trace the relevant Quarto book output; done means the index page renders without that TOC while the other pages remain unchanged.