jupyterlab-markdown-syntax-rendering-fix 0.6.9

Jupyterlab extension to fix a common issue with Markdown renderer where some race condition causes for the fenced code block to not have proper syntax highlighting

jupyterlab_markdown_syntax_rendering_fix

Fenced code blocks in rendered Markdown sometimes appear plain and uncoloured - the highlighting silently fails when a CodeMirror language chunk loads late or the language registry is not yet ready, and JupyterLab falls back to plain text. This extension restores the highlighting that was lost to that race.

Features

• Recovers lost highlighting - re-applies syntax highlighting to fenced code blocks that rendered plain because the async highlighter missed the cache
• Language agnostic - works for any fenced language (bash, python, json, ...), independent of mermaid
• Targets the cold-load race - handles the case where a language chunk imports late or the registry is wired after an early render
• Frontend only - pure TypeScript labextension, no server component

How it works

JupyterLab highlights fenced code in rendered Markdown with an async pass that fills a cache and a synchronous renderer that reads it. When the async highlight throws - a CodeMirror language-chunk import that rejects, or a registry that is not yet wired when an early render fires - the cache misses and the renderer emits a plain <pre><code>. This extension watches the application shell for those plain blocks and re-runs the highlight once the language is available.

• Detect - a MutationObserver flags any rendered pre > code that has a language-* class and text but no token <span> children
• Recover - re-runs the highlight through IEditorLanguageRegistry and swaps in the token spans, only when the highlighted text matches the source exactly so it never truncates content
• Resilient - retries a thrown highlight a few times with backoff while the language chunk finishes loading, then gives up cleanly and leaves the original plain text untouched
• Unobtrusive - each block is handled at most once, and editor and overlay churn is skipped to keep the observer cheap

Requirements

• JupyterLab >= 4.0.0

Install

To install the extension, execute:

pip install jupyterlab_markdown_syntax_rendering_fix

Uninstall

To remove the extension, execute:

pip uninstall jupyterlab_markdown_syntax_rendering_fix