Describe the bug

We have upgraded from v5.3.0 to v7.2.6 in pytest-dev/pytest#11568, but did not notice until recently that the search bar disappeared, while the search functionality (search.html?q= in the address bar) still worked.

Downgrading it back to v5.3.0 (without any other changes) brings the search bar back: pytest-dev/pytest#12000.

How to Reproduce

• Clone https://github.com/pytest-dev/pytest.
• Setup a virtual environment.
• Change the Sphinx pinning in doc/en/requirements.txt from sphinx>=5,<6 to sphinx>=5,<8.
• Install tox and execute tox -e docs.
• Open doc/en/_build/html/index.html locally - the search bar does not show up anymore.

Environment Information

Platform:              win32; (Windows-10-10.0.19045-SP0)
Python version:        3.12.2 (tags/v3.12.2:6abddd9, Feb  6 2024, 21:26:36) [MSC v.1937 64 bit (AMD64)])
Python implementation: CPython
Sphinx version:        5.3.0
Docutils version:      0.19
Jinja2 version:        3.1.3

Sphinx extensions

No response

Additional context

No response