Checklists in Rich Text fields have some quirks that make them unnecessarily cumbersome, compared to e.g. how Bulleted Lists work.
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When the cursor is at the beginning of a non-blank line, inserting a Checklist (by typing “/k ENTER”) creates a NEW empty Checklist BELOW the current line. It would be preferable to instead CONVERT the current line into a Checklist (as happens with Bulleted lists and everything else).
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When the cursor is at the end of the line on the last item of a Checklist, hitting DELETE does not append/convert the following line into the current line (last Checklist item) as expected.
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Checklist items/lines cannot be converted into other paragraphs types (headings, bulleted lists, etc) without first changing them into a regular paragraph.
 

 Check lists are not supported either by the original Markdown spec nor by “Commonmark”, one of the more, er, commonly used reference specs. But they are supported by the also widely-used “Github Flavored Markdown”, which I believe Fibery either uses, or at least conforms fairly close to. And that spec does indeed include nested checklists. 