I used to do manual HTML markup in my markdown, and it was working before, but I think it's broken in newer versions :(

As an example:

<details><summary>19: Variables</summary>
### 19: Variables

- ...
- ...

</details>

With the latest version (which I think would be: 0.11.8(2018/7/19)), I see something like this:

But clicking the arrow on the left doesn't do anything (as you may have guessed by noticing that the header (created by ###) is actually not inside the region but outside

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AFAIK, Boostnote supports header folding (tested in v0.12.1), in editor mode of course.

Is this what you suggest @FabianTe ?

What @farzadmf mentioned is a markdown parser problem, similar to this issue HTML Table does not render if there's blank line inside markup

Kind of. But I'd also like to be able to fold the rendered markdown headers (right side)