Isolated provider-authored markup rendering

Provider-authored Markdown is rendered as formatted content rather than printed as source text. One shared renderer serves release notes, pull request content, comments, summaries, and offline documentation. The renderer remains isolated from the privileges of the surrounding application.

Behavior

SandboxedMarkdown parses GitHub-flavored Markdown with line breaks, sanitizes the resulting HTML, embeds it in a sandboxed same-origin data-URL iframe, and loads the bundled local Markdown stylesheet. A base URL resolves relative links, the shared emoji map resolves supported shortcodes, and callers provide an accessible region label.

Only HTTP and HTTPS links reach the caller's navigation callback. Images are bounded to their container. Theme variables and the user's underline-links choice are copied into the isolated stylesheet. Provider-specific node filters run after the document is current and owned by the latest render generation.

The Linux terminal edition uses Textual's local Markdown widget for the same formatted headings, lists, links, and code outcome. Documentation source is converted into the static hub and offline bundle at build time, so pages remain readable without client-side Markdown execution.

Configuration

Callers provide Markdown, optional base URL, emoji map, provider context, repository identity, link callback, accessible label, underline preference, and optional trusted application-owned CSS. Provider content cannot supply CSS or executable script.

Failure modes and recovery

A missing bundled stylesheet degrades appearance without dropping content.
A superseded render cannot initialize or resize the newest document.
Unmount cancels animation-frame polling, document listeners, and debounced scroll work.
An unsafe link is prevented and is not sent to the navigation callback.
Empty provider content is represented by the owning surface's factual empty state instead of an empty loading frame.

Security considerations

HTML is sanitized before it enters the iframe. The iframe has no script permission. Provider-authored CSS is not accepted. Link handling validates the protocol. The renderer reads one bundled stylesheet and makes no network request. Repository and provider filters receive typed context rather than arbitrary application privileges.

Verification

sandboxed-markdown-lifecycle-test.tsx covers repeated reloads, latest-render ownership, deferred document initialization, unmount cancellation, link and tooltip setup ownership, and stale work refusal. Release-note and documentation wiring tests prove real consumers use the shared renderer instead of printing Markdown source. The terminal and static-page implementations are covered by their existing UI and documentation-generation tests.