Scope Markdown reader offsets by panel identity

[bb-slop-cop]

🚨 SLOP COP 🚨 · new-issue-autopilot

Human comments

What was wrong

Markdown reader offsets live in one panel instance even though the panel's tab list changes between threads. A different thread's tabs prune the previous thread's positions; remounting discards the map, while equal tab IDs can leak positions across threads.

Reproduction and root-cause report: https://get-bb.github.io/reports/issues/5615.html

What changed

Keep the in-session offset maps keyed by the existing panel-state identity. Preserve per-instance behavior for unscoped panels and keep closed-tab pruning within the current scope.

Extend the existing real-panel regression harness for disjoint tab lists, shared tab IDs, remounting, and scoped close/reopen behavior. Two secondary-panel files change; no new dependency, generated file, stored-data change, public API, or daemon protocol change.

Changed-line count against trusted origin/main: 83 text lines (69 additions + 14 deletions), 2 files.

How you verified

• Before the production fix, the focused suite fails with 3 failed / 6 passed at trusted main 01affc197dc621cddc7343e09b1c5c10a5b9efcb. The same agent repeats those failures in a second clean checkout at that exact commit.
• After the fix, 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 passes 72/72 across four files.
• pnpm exec turbo run typecheck lint --filter=@bb/app passes; lint reports 250 existing warnings and 0 errors, with none in the changed panel file.
• Isolated source app in Chromium: real sidebar navigation changes an open preview from scrollTop 6400 to 0 before the fix; the same sequence restores 6400 after the fix. The report includes inspected screenshots.
• git diff --check and the secret-model scan pass. No binary changes.

Fixes #5615

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