Mermaid diagrams don’t render in Gist editor preview (only after publishing)

[icedmoca]

Select Topic Area

Bug

Body

When creating or editing a Gist with Mermaid code blocks (```mermaid fences), the editor preview does not render the diagram. It only shows the raw code.

After saving/publishing the Gist, GitHub correctly renders the Mermaid diagram on the gist page.

---

Expected behavior

• The preview panel in the Gist editor should render Mermaid diagrams, just like it does for Markdown and code highlighting.

---

Why this matters

• Without preview support, users can’t verify if their diagrams are valid or formatted correctly before saving.
• This causes extra trial and error and breaks the normal preview workflow.

---

Steps to reproduce

1. Create a new Gist.
2. Add a Mermaid diagram block with ```mermaid.
3. Switch to Preview.
4. Diagram does not render.

---

Environment

• GitHub Gists (web UI).

---

Suggestion

Enable Mermaid rendering in the Gist editor preview, consistent with how GitHub repositories render Markdown with Mermaid.

[github-actions[bot]]

💬 Your Product Feedback Has Been Submitted 🎉

Thank you for taking the time to share your insights with us! Your feedback is invaluable as we build a better GitHub experience for all our users.

Here's what you can expect moving forward ⏩

• Your input will be carefully reviewed and cataloged by members of our product teams. 
  • Due to the high volume of submissions, we may not always be able to provide individual responses.
  • Rest assured, your feedback will help chart our course for product improvements.
• Other users may engage with your post, sharing their own perspectives or experiences. 
• GitHub staff may reach out for further clarification or insight. 
  • We may 'Answer' your discussion if there is a current solution, workaround, or roadmap/changelog post related to the feedback.

Where to look to see what's shipping 👀

• Read the Changelog for real-time updates on the latest GitHub features, enhancements, and calls for feedback.
• Explore our Product Roadmap, which details upcoming major releases and initiatives.

What you can do in the meantime 💻

• Upvote and comment on other user feedback Discussions that resonate with you.
• Add more information at any point! Useful details include: use cases, relevant labels, desired outcomes, and any accompanying screenshots.

As a member of the GitHub community, your participation is essential. While we can't promise that every suggestion will be implemented, we want to emphasize that your feedback is instrumental in guiding our decisions and priorities.

Thank you once again for your contribution to making GitHub even better! We're grateful for your ongoing support and collaboration in shaping the future of our platform. ⭐