Pin color-scheme on rich-part surface frames (fix white canvas in dark mode)

[benvinegar]

Problem

A markdown (or code/diff/mermaid) surface renders as a near-blank white box with washed-out text on a dark board:

Take 3 — code review. posts a diff surface; pin comments to lines like a PR.

…rendered as faint light-gray text on solid white, inside an otherwise-dark card.

Root cause

Rich-part surfaces (renderSandboxedPart, renderMermaidPage) are sandboxed, opaque-origin iframes loaded by URL. Such a frame defaults to color-scheme: normal (i.e. light). Its body is background: transparent — the design intends the dark card to show through — but with a light color-scheme the UA paints the iframe canvas white, and the dark-mode text (--text: #e6edf3) washes out over it.

A comment in renderSandboxedPart explicitly omitted color-scheme, reasoning that forcing it "would paint an opaque UA canvas." That's backwards: omitting it is what leaves the canvas light/white; html surfaces (renderHtmlPage) already pin it via colorSchemeCss(mode) and render correctly.

Verified by reproduction (Playwright, real engine + cloud embed, dark mode): the markdown frame's computed values are all "dark + transparent," yet it visibly paints white — and adding colorSchemeCss(doc.mode) makes it render correctly and seamlessly against the card.

before	after
white box, faint text	transparent, readable on the dark card

Change

• renderSandboxedPart and renderMermaidPage now emit colorSchemeCss(doc.mode) — the exact pin html surfaces already use — so the UA canvas tracks the resolved scheme (dark→dark, light→light). No mode → left to the OS, unchanged.
• Corrected the misleading comment.
• Updated the regression test: it previously asserted the buggy "rich frame must NOT force color-scheme"; it now asserts the pin (dark→:root{color-scheme:dark}, light→light, no-mode→unpinned).

Tests

npm test (400), npm run typecheck, npm run lint, npm run format:check all green.

🤖 Generated with Claude Code