Restore scrolling in Markdown secondary-panel previews

[brsbl]

Human comments

What was wrong

Markdown used a content-height layout that required a scrolling parent. Plugin file openers render the built-in preview inside a full-height, overflow-hidden region, leaving long documents without a scroll container. #5185's offset tracking did not change these layout classes.

What changed

• Markdown previews now own a bounded scroll container inside the secondary panel.
• The existing per-tab offset tracking saves and restores that container.
• A regression covers full-height opener hosts and returning from a shorter document.

How you verified

• Remote regression proof: test-only CI run fails with Missing file scroll container.
• Chrome 155.0.0.0: hard reload, wheel scroll to 800px, short-document switch, and return restore exactly 800px.
• Final remote CI passes: all six Markdown scroll tests, app tests, lint, and typecheck.
• Focused review at 561d36a found no P0–P2 issues; the final follow-up only formats the test harness.

Before: e9ad6b0 · After: 54ee42f · Captured at 2×.
Both captures use the same fixture, Preview tab, and 800px wheel input; before remains at zero.

Screen / state	Viewport	Before	After
Thread Markdown Preview after wheel input	Desktop · 1280×577
Thread Markdown Preview after wheel input	Compact web · 390×844

User QA approval is pending.

Preview ownership and fixture

Branch dev app remains running in terminal term_pbpi7vf66x on host host_57pj4zhqxr, environment env_unhfrbci6j, from this PR worktree via pnpm dev. App port 15456, server port 23456, daemon port 31456. Fresh dev data; synthetic long and short documents; a minimal file opener delegates to Original. This reproduces the same full-height opener path without needing a production plugin or personal data.

Fixes #5336

BB-Thread-ID: thr_frq37qj86d

AGENT GENERATED

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-10T16:53:49.211933Z	54ee42f	PR opened

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