Right sidebar tab splits: shared header and resizing

[brsbl]

What was wrong

The production split foundation treated adjacent sidebar tab groups too much like full page panes: its chrome and separators competed with the existing right-panel header, top/bottom placement complicated the shared header, tab headers could visually drift from their bodies while resizing, and overflow lacked a clear containing strip.

What changed

• Kept one existing right-panel header and presents split groups as tabs side by side inside it.
• Limited the reused hover arrangement menu to the supported left/right positions while preserving full-screen.
• Uses sanctioned subtle seam tokens for adjacent tab groups, keeps stronger page/sidebar dividers unchanged, and matches horizontal thread split seams to the thread-header seam.
• Synchronizes header and body widths continuously during drag resizing, contains horizontal tab overflow, preserves one add-tab action and the far-right collapse control, and retains existing active/inactive split treatment.
• Keeps data and terminal lifecycles correct when focus moves across visible split panes, and cancels resize state if a divider loses capture or unmounts mid-drag.
• Preserves unsplit tabs and unrelated panel behavior.

How you verified

• pnpm exec turbo run test --filter=@bb/app --force -- src/components/secondary-panel/SidebarSplitContainer.test.tsx src/components/secondary-panel/sidebarSplitLayout.test.ts src/components/secondary-panel/ThreadSecondaryPanel.collapseControl.test.tsx src/components/secondary-panel/SecondaryPanelTabStrip.test.ts src/views/thread-detail/SplitThreadArea.test.tsx src/components/thread/terminal/useThreadTerminalController.test.ts — 97 tests passed.
• pnpm exec turbo run typecheck --filter=@bb/app — passed.
• pnpm exec turbo run lint --filter=@bb/app --force — passed with 147 pre-existing warnings and no errors.
• git diff --check origin/bb/sidebar-split-panes-thr_4rr623umv4...HEAD — passed.
• Exact-head desktop QA at 3406da4a6c6f2cecf3f2fb9eb2d82ec150b6a2d2 confirmed a single unchanged header; Browser and Side chat side by side; active-pane targeting; synchronized header/body geometry during inner drag; independent outer resizing; preserved full-screen behavior; and matching split seams in light and dark themes.

Before — parent foundation 3ad3d0f6c36b0b9b581360d0ca56d1c342b2b1a3

Viewport 1440 × 900 at DPR 2; image 2880 × 1800.

After — final child 3406da4a6c6f2cecf3f2fb9eb2d82ec150b6a2d2 (light)

Viewport 1440 × 900 at DPR 2; image 2880 × 1800.

After — final child 3406da4a6c6f2cecf3f2fb9eb2d82ec150b6a2d2 (dark)

Viewport 1440 × 900 at DPR 2; image 2880 × 1800.

Fixes #1841

BB-Thread-ID: thr_7mjwczzstc

AGENT GENERATED: by GPT-5

[bb-slop-cop]

🚨 SLOP COP 🚨 · review

I am SlopCop, and I am reviewing this pull request now. I will check security, code quality, performance, architecture, and the main UI flow.

[bb-slop-cop]

🚨 SLOP COP 🚨 · review

Plain-English summary

This PR gives split right-sidebar panes one shared tab bar. It limits new tab splits to left and right. It also tries to keep each visible terminal mounted across panes.

Findings

I found three defects and left two inline comments.

• High: A visible terminal in an unfocused pane can lose its tab after a disconnect. The new retained-ID list does not reach the global tab synchronization paths.
• Medium: Stored vertical layouts remain valid. Their horizontal shared header no longer matches their vertically stacked bodies.
• Low: Each terminal pane creates a new retained-ID array. This restarts disconnected-session cleanup after unrelated parent renders.

I found no security issue.

The architecture scan found duplicate panel and file-preview renderer paths. They increase maintenance cost, but they do not create a separate defect in this PR.

Verification

• The @bb/app Turbo type check passed.
• All 75 focused tests passed.
• git diff --check passed.
• The browser test created a two-pane horizontal split and verified the shared header.
• The browser test restored a vertical version-1 layout. It measured horizontal header groups and vertically stacked bodies.

I used a GPT-5.6 review gate. It confirmed all three findings.