[kilhyeonjun]

Change

Completed Mermaid fences in chat can show a diagram, with Source/Diagram tabs, original-source copy and a native expanded dialog with zoom and fit. Streaming, malformed, unsupported, oversized or unavailable diagrams retain the existing code view. Ordinary code fences keep their current behavior.

This is a focused Mermaid-only contribution for #782, alongside the broader Mermaid/images work in #616. No dependency on that branch. Opening as a draft for the maintainer's feature/dependency decision.

Mermaid 11.16.1 is pinned and lazy loaded; a scoped KaTeX override uses the app's existing 0.18.9. The application fixes strict configuration, refuses input configuration/HTML/custom CSS/resource directives, serializes initialize/render/owned cleanup, and validates bounded SVG before displaying a Blob image. Source, pending work and cache are bounded. Rendering still uses temporary DOM on the main thread; these guards do not guarantee interruption or complete execution isolation.

Includes ko/ja/zh labels, library and bundled runtime notices, documentation, changelog and registered browser regression with synthetic screenshots.

Validation

• Generated schema freshness, production build, full typecheck: pass. New browser fixture/entrypoint explicitly typechecked too.
• Full unit suite: 2,511 pass, 50 skip, 0 fail (140.31s). Browser-lane registry test includes the new script.
• Actual pinned-library browser regression: lazy/unfinished fences; four diagrams and Korean; source/copy; light/dark; native modal focus/Escape; mobile and a >7,000px diagram's fit/zoom-out/0; rejected resources/SVG; delayed import with changed source, unmount and failed import; Blob/container cleanup. No unexpected external or same-origin probe requests observed in that fixture.
• Existing math browser regression: pass (KaTeX first-expression lazy load, inline/display and offline source fallback).
• bun audit: no vulnerabilities found in 260 packages.
• Independent design/implementation review; mobile fit and zoom-out findings fixed and regression tested.
• Workflow syntax check skipped: actionlint is unavailable; no workflow YAML changed. Broader Native lanes are not claimed passing: the unchanged upstream baseline is being compared with existing server-contract failures. Fork CI still needs maintainer approval.

Synthetic screenshots: mobile diagram, expanded dialog.

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[devswha]

Thanks for this. Mermaid diagrams in chat are already under review in #616, which the maintainer has reviewed, and as the description says, this PR is a focused alternative to it. Closing as a duplicate to keep the work in one place. If the bounded rendering here (size limits, source fallback, the refused config/HTML input) covers cases #616 misses, a comment on #616 with those cases would be very welcome.