Short version:
<details><summary>...</summary>
<p>
<!-- following line break is intentional otherwise the code block wont render + don't forget to use triple backticks! -->
``bash
#
``
</p>
</details>
Long version:
<details><summary>CLICK ME</summary>
<p>
The line break after the paragraph is intentional.
Without it the rendering of the below code block fails.
#### yes, even hidden code blocks!
Don't forget to use triple backticks, I've used two for the purposes of rendering this code within a Markdown formatted gist...
``python
print("hello world!")
``
...only using two for sake of displaying nested code block
</p>
</details>