The sidebar does not expand when the ToC depth is limited

[petsuter]

Describe the bug
The sidebar does not expand when the ToC depth is limited.

To Reproduce
Steps to reproduce the behavior:

• sphinx-quickstart and select all defaults (Alabaster theme)
• Change / create these files:
• index.rst:

 .. toctree::
   :maxdepth: 1
   
   sub/index

• sub/index.rst:

Sub
===

.. toctree::
   :maxdepth: 1
   
   topic

• sub/topic.rst:

Subtopic
========

test

$ make html

• Open _build/html/index and see everything looks fine. The sidebar on the left shows "Sub". Also the ToC in the middle shows "Sub". All looks OK.
• Click "Sub" and see ToC in the middle shows "Subtopic" as expected. But the problem is the sidebar still only shows "Sub". It did not expand and does not show "Subtopic"!

test-sphinx.zip
sphinx-doc/sphinx#7171

Expected behavior
The sidebar should expand, even if :maxdepth: is used.

Environment info

• OS: Windows 10
• Python version: 3.7.1
• Sphinx version: 2.4.1
• Sphinx extensions: None

[petsuter]

This is still a problem in Sphinx 3.1.1 / Alabaster 0.7.12