Nested dropdown not showing down-chevron

[stefanodavid]

Describe the bug

A collapsed nested dropdown will not show the "down-chevron" to expand its content.

However, the "up-chevron" to collapse the dropdown is shown correctly when the dropdown is expanded.

To Reproduce

Copy and paste the following rst code and compile

.. dropdown:: a dropdown

   sample dropdown

   .. dropdown:: a nested dropdown
                 
      sample nested dropdown w/o chevron

Expected behavior

A down-chevron should be displayed to make clear there is some hidden content.

Environment

• Python 3.7.3 (on Debian Buster)
• sphinx-panels 0.5.2
• theme: I tried with both alabaster & rtd

Additional context

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[chrisjsewell]

This will essentially be closed by https://github.com/executablebooks/sphinx-design, where it has already been fixed