Idea: render settled mermaid fences as diagrams in the web GUI

[MarchLiu]

Summary

Assistant replies carrying ```mermaid fences currently render as plain code blocks in the web GUI. The fence language already declares the intent — rendering the diagram in place would save a copy-paste round trip to an external renderer.

Proposal

Add a settled-only mermaid branch to the markdown renderer's fence handling (packages/client/ui-primitives, mirroring the existing math fence precedent):

• Load mermaid behind a dynamic import (~500 KB stays out of the boot chunk; sessions without diagrams never pay it).
• Initialize once with securityLevel: 'strict' — fence source is untrusted assistant output, and mermaid DOMPurify-sanitizes labels before emitting the SVG.
• Validate with mermaid.parse before mermaid.render; any failure (chunk load, parse, or render) degrades to the source CodeBlock instead of throwing, matching the unknown-grammar fallback rule.
• While streaming, the fence stays a plain CodeBlock so the frozen-block streaming cache never bakes in async work.
• Theme: mermaid's built-in neutral palette. (base + themeVariables mapped to --dsw-* tokens was rejected: mermaid's color math cannot parse CSS var() references, and literal colors would violate the tokens-only styling rule and freeze diagrams against light/dark switches.)

Working implementation

I have this implemented and fully gated on a fork branch: feat/mermaid-fence-rendering (reviewable diff: MarchLiu/deepseek-harness#1).

• Unit tests mock the mermaid module and pin the loading/fallback contract; MermaidBlock.tsx and render.tsx hold 100% line/branch coverage; the byte-pinned markdown DOM-parity fixtures are untouched.
• A keyless assembled-browser e2e scenario (apps/web/tests/mermaid-fence.e2e.ts, hand-authored session fixture + Playwright Chromium + aria golden) proves the diagram replaces the source CodeBlock in the real app with no console warnings.
• Full local ladder green: test:gui (273 files / 3765 tests), lint, doc-sync (28 gates), duplication, hygiene, DSH_SNAPSHOT=replay test:web (76 files / 255 tests), and the full-repo vitest run --coverage gate (804 files / 13411 tests). An Agent Note in the repo's format is included.

I understand external pull requests are not accepted at the moment — posting this here in case the team wants to pick the idea (or the branch) up. Happy to adjust the design (theme mapping, error affordance, rendering site) if a different direction fits better.