Issue with Markdown Rich Text Editor (Slate) and headings

[Blaec-CoCalc]

After creating a heading, you cannot type anything in the rich text editor of a markdown file.

To reproduce:

1. Open a Markdown file
2. Create a ##### heading and populate it.
3. Hit enter to make a new line.
4. Try to type something underneath (for example, try to make a bullet point)
5. It should be non-responsive.

[williamstein]

I cannot reproduce this.