Collapsible Headers (Notion/Google Docs-style)

[ugosan]

Have you read a contributing guide?

• I have read CONTRIBUTING.md
• I have searched the existing requests and didn't find any that were similar
• I have considered creating a pull request instead and want to proceed

Clear and concise description of the problem

I have implemented Collapsible Headers as described here

Behavior
H1 collapses everything until the next H1
H2 collapses everything until the next H2 or H1
H3 collapses everything until the next H3, H2, or H1
Arrow shows at end of header line, hidden when expanded (visible on hover), always visible when collapsed
State is persisted, defaults to expanded

Benefits
Better navigation in long documents
Familiar UX from Notion and Google Docs
Drastically reduces visual clutter

Suggested solution

I am creating collapsible headers (H1, H2, H3) that work like Notion and Google Docs.

Alternative

No response

Additional context

No response

[ra3orblade]

This is implemented in latest build differently and works the same way it works in Notion.