Enhancement: Support configurable outline depth and collapsible sections in View Outline

[leonardjjw]

Description

Hi marimo team,

Thank you for building such a great reactive notebook environment!

I’d like to suggest two improvements to the View Outline sidebar:

Configurable heading depth:

Currently, the outline only shows headings up to level 3 (###). It would be very helpful—especially for long or deeply structured documents—if users could configure the maximum heading level shown (e.g., max_level: 4 or 6) via marimo.json or app settings.
Collapsible outline sections:

It would greatly improve navigation if headings in the outline were collapsible (like in VS Code or JupyterLab). For example, clicking a ▶/▼ icon next to a level-2 heading would expand or collapse its child headings.
These features would make it much easier to navigate complex notebooks while keeping the UI clean and organized.

Thanks for considering this enhancement!

Suggested solution

I’d like to see two enhancements to the View Outline feature:

Configurable outline depth:

Add a setting (e.g., outline.max_level in marimo.json) to control how deep heading levels appear in the outline. For example:
json edit
{
"outline": {
"max_level": 4
}
}
This would allow #### Level 4 headings (and deeper, if desired) to appear in the sidebar.
Collapsible sections in the outline:

Make each heading in the outline collapsible via a toggle icon (▶/▼). Clicking it would expand or collapse its child headings—just like in VS Code or modern IDEs.
User experience improvement:

In a notebook with many sections and subsections, users could now:

See fine-grained structure (e.g., methods under a class, or steps under a phase)
Fold away parts they’re not currently working on
Navigate large documents more efficiently without scrolling endlessly
These changes would make marimo much more powerful for technical documentation, data reports, and educational content.

Are you willing to submit a PR?

• Yes

Alternatives

No response

Additional context

No response