Add mermaid diagram support to markdown preview

[ghost]

Enable rendering of mermaid diagrams in markdown preview, allowing users to create flowcharts, sequence diagrams, and other visualizations directly in markdown files using standard mermaid syntax.

• Added ParsedMarkdownMermaidDiagram element type to handle mermaid code blocks
• Created mermaid_renderer module that uses @mermaid-js/mermaid-cli (v11.4.0)
• Integrated async rendering pipeline into markdown preview workflow
• Renders diagrams as high-resolution PNG (2x scale) with transparent background
• Automatically installs mermaid-cli via npm on first use

flowchart TD
      A[Markdown File] --> B{Parser}
      B -->|mermaid blocks| C[ParsedMarkdownMermaidDiagram]
      B -->|Other blocks| D[Other Elements]
      C --> E[Async Render Pipeline]
      E --> F[Get NodeRuntime]
      F --> G[Install mermaid-cli]
      G --> H[Execute mmdc]
      H --> I[Generate PNG]
      I --> J[Display in Preview]
      D --> J

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Technical Details

Modified files:

• crates/markdown_preview/src/markdown_elements.rs - New diagram element type
• crates/markdown_preview/src/markdown_parser.rs - Detect mermaid blocks
• crates/markdown_preview/src/markdown_renderer.rs - Render diagram images
• crates/markdown_preview/src/mermaid_renderer.rs - New: mermaid integration
• crates/markdown_preview/src/markdown_preview_view.rs - Async rendering pipeline
• crates/markdown_preview/src/markdown_preview.rs - Module declaration
• crates/markdown_preview/Cargo.toml - Added dependencies

Rendering options:

• Theme: neutral (for clean, simple output)
• Background: transparent (adapts to light/dark themes)
• Scale: 2x (high-resolution for retina displays)
• Format: PNG (universal compatibility)

Cross-Platform Support

Works on macOS, Windows, and Linux. Linux users may need additional system
packages for Puppeteer (libx11-xcb1, libxcomposite1, etc.).

Example Usage

graph LR
    A[Start] --> B[Process]
    B --> C[End]

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Release Notes:

• Add mermaid diagram support to markdown preview.

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Example:

[Angelk90]

@oscarvarto: I tried, when I edit the code, it doesn't update automatically, does this happen to you too?

Registrazione.schermo.2025-11-16.alle.12.23.21.mov

I also tried the code below, it looks like this.

```mermaid
flowchart TD
      A[Markdown File] --> B{Parser}
      B -->|mermaid blocks| C[ParsedMarkdownMermaidDiagram]
      B -->|Other blocks| D[Other Elements]
      C --> E[Async Render Pipeline]
      E --> F[Get NodeRuntime]
      F --> G[Install mermaid-cli]
      G --> H[Execute mmdc]
      H --> I[Generate PNG]
      I --> J[Display in Preview]
      D --> J

[ghost]

Yesterday I worked so much that today I need a break. Will revisit this probably next weekend

[ghost]

@Angelk90: I've addressed the re-rendering issue, it should be working now, with parallel rendering, caching, and scaling. Copying commit message:

Optimize mermaid diagram rendering with smart caching and configurable scaling

Add intelligent content-aware caching and parallel rendering to dramatically
improve performance when editing markdown files with mermaid diagrams. Also
introduce configurable per-diagram scaling via fence syntax.

Performance Improvements

Smart Content-Based Caching

• Compute hash of diagram content + scale to detect changes
• Only re-render diagrams whose content actually changed
• Cache rendered PNGs with LRU eviction (max 50 diagrams)
• Reuse cached diagrams instantly when content unchanged

Performance impact:

• Editing non-mermaid content: 0 diagrams re-rendered (instant updates)
• Editing one diagram: only that diagram re-renders (~1s vs 5-10s before)
• Switching between cached content: instant retrieval

Parallel Rendering

• Render multiple diagrams concurrently using futures::future::join_all
• 2-3x faster for documents with multiple diagrams
• Each diagram renders in independent background task

Configurable Scaling

Users can now specify scale percentage in the code fence:

graph LR
    A --> B

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Syntax: ```mermaid [scale] where scale is 10-500 (default: 100)

Scale calculation:

• 50 → scale factor 1 (50% of retina default)
• 100 → scale factor 2 (default retina)
• 150 → scale factor 3 (1.5x larger)
• 200 → scale factor 4 (2x larger)

Smart caching: Different scales create separate cache entries, so changing
scale triggers re-render but switching back to previous scale uses cache.

Architecture

flowchart TD
    A[Edit Markdown] --> B{Parser}
    B -->|mermaid block| C[Compute Hash]
    C --> D{In Cache?}
    D -->|Yes + File Exists| E[Reuse Cached PNG]
    D -->|No| F[Spawn Render Task]
    F --> G[Multiple Tasks in Parallel]
    G --> H[Execute mmdc]
    H --> I[Save to Cache]
    I --> J[Update LRU]
    J --> K{Cache > 50?}
    K -->|Yes| L[Evict Oldest]
    K -->|No| M[Display]
    L --> M
    E --> M

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Technical Details

Modified files:

• markdown_elements.rs - Added scale: u32 and content_hash: Option<u64> fields
• markdown_parser.rs - Parse scale from fence syntax, compute content hash
• markdown_preview_view.rs - Smart cache with LRU, parallel rendering
• mermaid_renderer.rs - Accept scale parameter, use integer for mmdc
• Cargo.toml - Added futures dependency

Hash computation:

let content_hash = {
    let mut hasher = collections::FxHasher::default();
    code.hash(&mut hasher);
    scale.hash(&mut hasher);  // Scale affects hash
    hasher.finish()
};

LRU cache eviction:

• Max cache size: 50 diagrams
• Evicts least recently used entries
• Deletes corresponding PNG files on eviction

Parallel rendering:

let render_tasks: Vec<_> = diagrams_needing_render
    .map(|diagram| cx.background_executor().spawn(...))
    .collect();
let results = futures::future::join_all(render_tasks).await;

Scale Parameter Details

Limits:

• Minimum: 10% (prevents too-small diagrams)
• Maximum: 500% (prevents excessive memory usage)
• Default: 100% (2x scale factor for retina displays)

Note: mermaid-cli requires integer scale values, so:

• Values 10-49 → scale factor 1
• Values 50-99 → scale factor 1
• Values 100-149 → scale factor 2
• Values 150-199 → scale factor 3
• etc.

Breaking Changes

None. Existing ```mermaid blocks without scale work exactly as before
(default 100% = scale factor 2).

Testing

Test cases to verify:

1. ✅ Edit non-mermaid content → no diagram re-renders
2. ✅ Edit one diagram → only that diagram re-renders
3. ✅ Multiple diagrams → render in parallel
4. ✅ Undo diagram edit → uses cached version
5. ✅ Scale 50, 100, 150, 200 → correct sizes
6. ✅ Change scale → triggers re-render with new hash
7. ✅ Cache exceeds 50 diagrams → LRU eviction works

Example Usage

Small flowchart:
```mermaid 50
graph LR
    A --> B
```

Normal sequence diagram:
```mermaid
sequenceDiagram
    Alice->>Bob: Hello
    Bob->>Alice: Hi
```

Large complex diagram:
```mermaid 200
flowchart TD
    A[Start] --> B{Decision}
    B -->|Yes| C[Process]
    B -->|No| D[Skip]
    C --> E[End]
    D --> E
```

[Angelk90]

@oscarvarto : The text in the center cannot be read.
You need to consider the themes.

[ghost]

@Angelk90 You can customize your diagram with styles, did you know? I appreciate the feedback. A friendlier phrasing like 'could you consider theme support?' would be more appropriate for volunteer work.

[bennetbo]

Hey, thanks for taking the time to work on this. I think it's not ideal to download an npm JS package for parsing Mermaid during render and include this out of the box.
This feels like something we'd like to support via Extensions, although our Extension API is not powerful enough yet to do something like this.
Also, as we're planning to unify the markdown and markdown_preview crates soon, I'd prefer to avoid adding any more complexity for now. Hopefully we can revisit this in the future. For now i'll close.