[Bug] Mermaid rendering: lines cross boxes

[alexpGH]

Description

The mermaid rendering draws line through boxes.

Environment

• OS: Debian
• Ferrite version: 0.2.6-hotfix1
• Rust version:
• Installation method: pre-built binary

Steps to Reproduce

1. add the following code and see preview:

graph TD
    linkStyle default interpolate basis
    A[Enter Chart Definition] --> B(Preview)
    B --> C{decide}
    C --> D[Keep]
    C --> E[Edit Definition]
    E --> B
    D --> F[Save Image and Code]
    F --> B

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flowchart TD
    A[Christmas] -->|Get money| B(Go shopping)
    B --> C{Let me think}
    C -->|One| D[Laptop]
    C -->|Two| E[iPhone]
    C -->|Three| F[fa:fa-car Car]

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[alexpGH]

Mermaid live and github renders the same, while Ferrite renders differently (I think you do actually not use themermaid library?) and produces an a bit awkward look.

[OlaProeis]

Good catch, and you're absolutely right - we don't use the Mermaid JS library. Ferrite has its own native Mermaid renderer built from scratch in Rust (using egui's drawing primitives), which means we're essentially reimplementing the layout and rendering logic ourselves. The upside is zero JavaScript dependencies and it works offline; the downside is exactly what you're seeing - our edge routing isn't as sophisticated as the official Mermaid library yet.

Specifically, the issue here is with edge routing. Our current implementation uses fairly simple path calculation that doesn't account for node bounding boxes when routing edges, so lines can cross through nodes instead of going around them. The Sugiyama-based layout algorithm we use handles vertical layering, but the edge path routing between layers needs improvement.

This is a known limitation we're actively working to improve. The fix involves implementing proper edge-node collision avoidance in the rendering pass. It's on the roadmap but I don't have an ETA yet since the edge routing logic is non-trivial to get right.

Thanks for the detailed examples and screenshots - they're great test cases for when we work on this!