[opencode-agent]

Clicking a #heading link in a Markdown preview (file viewer, and Markdown in messages) did nothing: the link was rendered as an external target="_blank" link, headings carry no ids, and the desktop window-open handler drops renderer URLs.

• marked-base: fragment-only links render as plain in-page links (no external-link class, target, or link icon).
• New setupFragmentLinks on the Markdown root: on click it resolves the fragment against that document only — headings by their GitHub-style slug (repeats as -1, -2), then sanitized raw HTML id/name (user-content- prefix) — and scrolls the nearest scroll container to it. The app URL and hash never change, and a missing target is a no-op instead of a navigation.
• Scrolls the scroller directly rather than scrollIntoView, which also scrolled a clipped ancestor in the file panel and pushed the Preview/Source toolbar up ~12px.

Testing

• bun test for slugging/target lookup and the parser output.
• New real-browser component test: clicking [Host services](#host-services) and a repeated-heading link #usage-1 lands each heading at the top of the scroller without changing the URL; fails without the fix.
• Full markdown.spec.ts component suite passes (37).
• Checked in the app's file viewer (e2e harness) with a dynamic-modules.md-style table: the link scrolls to the "Host services" heading and the toolbar stays put.

Visual

<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 240 72" width="240" height="72" fill="none">
  <style>
    .doc{animation:jump 3s ease-in-out infinite}
    .hit{animation:hit 3s ease-in-out infinite}
    @keyframes jump{0%,30%{transform:translateY(0)}45%,85%{transform:translateY(-52px)}100%{transform:translateY(0)}}
    @keyframes hit{0%,20%{opacity:.25}28%,32%{opacity:1}45%,100%{opacity:.25}}
    @media (prefers-reduced-motion:reduce){.doc,.hit{animation:none}}
  </style>
  <rect x="60" y="4" width="120" height="64" rx="6" stroke="currentColor" stroke-opacity=".3"/>
  <clipPath id="c"><rect x="61" y="5" width="118" height="62" rx="5"/></clipPath>
  <g clip-path="url(#c)">
    <g class="doc" fill="currentColor">
      <rect x="70" y="14" width="44" height="4" rx="2" opacity=".35"/>
      <rect class="hit" x="118" y="14" width="40" height="4" rx="2"/>
      <rect x="70" y="24" width="96" height="3" rx="1.5" opacity=".2"/>
      <rect x="70" y="32" width="80" height="3" rx="1.5" opacity=".2"/>
      <rect x="70" y="40" width="90" height="3" rx="1.5" opacity=".2"/>
      <rect x="70" y="48" width="70" height="3" rx="1.5" opacity=".2"/>
      <rect x="70" y="64" width="56" height="6" rx="2" opacity=".8"/>
      <rect x="70" y="76" width="96" height="3" rx="1.5" opacity=".2"/>
      <rect x="70" y="84" width="84" height="3" rx="1.5" opacity=".2"/>
      <rect x="70" y="92" width="92" height="3" rx="1.5" opacity=".2"/>
      <rect x="70" y="100" width="64" height="3" rx="1.5" opacity=".2"/>
      <rect x="70" y="108" width="88" height="3" rx="1.5" opacity=".2"/>
    </g>
  </g>
</svg>

Requested by: @Brendonovich (Brendan via Slack)

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Added by the opencode bot (AI-generated).

Fixes #57971

[opencode-agent]

The fix works. Your new browser test fails on the base (the fragment link still opens with target="_blank") and passes with this change, along with the rest of markdown.spec.ts (37). The fragment-link and parser unit tests pass, and tsc is clean for session-ui and ui. The user-content- fallback matches the sanitizer's existing SANITIZE_NAMED_PROPS setting.

Public API: unchanged. The new functions are internal to session-ui, and the marked-base link output changes only for #… links.

No blocking concerns. The new tests are small, and the browser test waits with expect.poll, not sleeps.

Fixes #57971