Mermaid blocks never render in the web editor's Preview tab (spinner forever); committed file renders fine

[codingoutloud]

🏷️ Discussion Type

Bug

Body

Mermaid blocks never render in the web editor's Preview tab: a spinner shows forever. The same content renders once committed.

Repro (public repo: https://github.com/codingoutloud/GitHub-render-bug-repro)

1. Open https://github.com/<owner>/<repo>/new/main and name the file t.md.

2. Content:

   ```mermaid
   flowchart LR
   a --> b
   ```
   

3. Click Preview. A loading spinner shows indefinitely and no diagram appears.

4. Commit the file and open it. The blob view's Preview renders the diagram correctly (t.md).

The same thing happens when editing an existing .md file (/edit/<branch>/<path>.md) that contains Mermaid and renders fine in its blob view. Reloading the page does not help.

What the DOM shows (Chrome DevTools): in the editor's Preview, each block's <section class="js-render-needs-enrichment render-needs-enrichment ... mermaid"> is never enriched. No viewscreen.githubusercontent.com/markdown/mermaid iframe is created, and the js-render-enrichment-loader spinner stays. The console shows no errors from GitHub. In the blob view, the iframes are created and every diagram draws.

Expected: Preview renders Mermaid the way the blob view does.

Environment (checked 2026-10-21):

• Chrome 155 on macOS, in a private org repo: Preview spins; the blob view renders.
• Safari on macOS, in the public repo above: Preview spins on the minimal flowchart; the committed blob view renders.
• Two browsers, two repos, different extension sets, so this does not look like an extension or a repo setting.

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