Retain Markdown reading offsets for open panel tabs

[bb-slop-cop]

🚨 SLOP COP 🚨 · new-issue-autopilot

Human comments

What was wrong

The secondary panel renders only the active file surface. Activating a browser surface removes the file's scroll container, and returning recreates it at offset zero. Switching to a shorter document can also clamp the shared wrapper. No scroll state outlives the ready Markdown body, even though its tab remains open. See the verified reproduction report.

What changed

• Cache ephemeral positions by open panel tab ID and prune closed-tab entries.
• Save native scroll events while the ready Markdown body is mounted; restore in its layout effect, not against a loading skeleton.
• Remove listeners without saving during outgoing cleanup, avoiding layout-clamped offsets.
• Add focused panel regressions for tab and hyperlink returns, separate document positions, delayed loading, and close/reopen cleanup.

The patch stays in the existing secondary-panel subsystem: 4 files, 284 additions + 6 deletions = 290 changed text lines. No dependency, stored-data, public API, CLI, protocol, or schema change is required.

How you verified

• Authored the regression before the production fix. On trusted main, four reading-position tests fail with expected +0 to be 780; the close/reopen control passes. Repeated in a second clean checkout, then refreshed onto 178fd3c2c4aeef162dea517d850ead11f3b034df and reproduced again.
• After the fix, 67 tests pass across the new regression and the existing file preview, panel collapse, and Markdown image-routing suites.
• Native Chrome source-UI reproduction: 4 failures / 1 control pass before; all 5 scenarios pass after, including a real long-to-short layout clamp and restoration after loading.
• pnpm exec turbo run test --filter=@bb/app -- src/components/secondary-panel/ThreadSecondaryPanel.markdown-scroll.test.tsx src/components/secondary-panel/ThreadSecondaryPanel.collapseControl.test.tsx src/components/secondary-panel/FilePreview.test.tsx src/components/secondary-panel/ThreadSecondaryPanelTabContent.markdown-images.test.tsx
• pnpm exec turbo run typecheck lint --filter=@bb/app passes; lint retains existing warnings.
• git diff --check passes; the fix has no binary changes.

The browser harness uses real source components with deterministic file retrieval and a local browser-surface activation callback. Installed desktop and Safari verification are not claimed.

Fixes #5181

AGENT GENERATED