fix(slides): disable scroll view on mobile to prevent crash

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📝 Summary

Serving a slides notebook on mobile (e.g. a WASM export opened on a phone) crashes with an error boundary:

undefined is not an object (evaluating 'this.slideTriggers[this.slideTriggers.length-1].range')

Root cause

reveal.js automatically switches to its scroll view when the presentation width drops below scrollActivationWidth (default 435px, i.e. most phones). Scroll view rewrites the slide DOM — it wraps every <section> in .scroll-page nodes and removes the .stack wrappers.

But marimo renders the deck via @revealjs/react, which owns those same nodes and re-runs reveal.sync() whenever the slide structure changes (documented behavior: slides added/removed/reordered/regrouped). During a WASM notebook load, cell outputs stream in and repeatedly change the composition, so sync() fires while scroll view has relocated/removed the nodes. reveal then re-runs syncPages() with zero .scroll-page elements, leaving slideTriggers empty, and setTriggerRanges() dereferences slideTriggers[-1].range → crash.

This is a structural incompatibility, not a tuning issue: the official React wrapper has an open bug where view: 'scroll' collapses the deck into a single slide, and scroll view also breaks deck.slide()/hash navigation which marimo relies on for active-cell sync.

Fix

Set scrollActivationWidth: 0 in the deck config to disable reveal's responsive scroll view. Mobile now keeps the same paginated deck as desktop; reveal's touch navigation (swipe) is on by default, so phone users still navigate naturally. 0 is used rather than null because the installed RevealConfig types the option as number.

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✅ Merge Checklist

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• Tests have been added for the changes made.

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	Jul 17, 2026 1:05pm

[Copilot]

Pull request overview

Disables reveal.js’s mobile “scroll view” mode for the slides deck to avoid a crash caused by reveal.js rewriting the slide DOM that @revealjs/react manages.

Changes:

• Set scrollActivationWidth: 0 in the reveal.js config to prevent auto-switching to scroll view on mobile.
• Document why scroll view is disabled (DOM ownership conflict leading to reveal.sync() crashes).

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}

[mscolnick]

I wonder if this will fix #10159

[github-actions]

🚀 Development release published. You may be able to view the changes at https://marimo.app?v=0.23.15-dev27