feat(split): independent split halves that may show the same tab

[devswha]

Change

The split view's two halves are now independent, as in the Claude desktop app's split (a pick in its sidebar replaces the focused pane) and the way Paseo and T3 Code keep panes put:

• A pick in the sidebar or the Agents list opens in the active half only. It never moves the focus to the other half and never trades the halves, also when the picked pane is the one the other half shows.
• Both halves may show the same tab. Slot b draws on a WebSocket of its own to the PC, so each half attaches and detaches the tab's panes by itself; closing one half no longer takes the other half's attachment with it.
• While both halves show one tab, the active half sizes the pty and the other half draws it at that grid (PaneFollowContext): it adopts pane-geometry, never resizes, skips every local fit, and its xterm sends no input or terminal query replies (a second reply reached the shell as typing). A press or key in that half makes it active first, so typing is unaffected.
• More drag sources open a half: an Agents row, a pane's title (a real mouse drag), and a half's own bar, beside the workspace row and tab that already did. A drop opens the pane in the half under it and makes that half active; nothing trades sides.
• A saved split whose other pane is not in the cached roster yet asks the PC (fetchSession) before it ends, instead of ending at load.
• The browser demo's transport now sends pane-geometry like the bridge (on a grid change, and the current grid to a keep_size/observe attach), so a following half matches on the published demo.
• docs/guide.md describes the new rules; changelog fragment changelog.d/split-view-independent-halves.md.

Known follow-up, not in this PR: a pending message owned by the other half's connection shows a "Discard saved copy" action in this half that does nothing (nothing is sent twice or lost).

Validation

All on this tree (ece0896 + this change), on a herdr of the run's own:

• bun run check fast: typecheck, build, 3825 unit tests pass, 0 fail.
• bun run check integration browser: integration 520 pass; the browser lane passes every script up to narrow-pane-demo-regression.ts. Locally three tests fail identically on a clean ece08969 checkout (api.contract "POST /api/pane/image > stores any other file under its own sanitised name", codex.contract same-byte-size invalidation, narrow-pane "a split under 80 columns"), while CI on main is green at that commit, so they are this machine's environment. The 8 scripts after narrow-pane, which the lane stops before locally, were run one by one: all pass.
• scripts/split-view-regression.ts alone on a live herdr: 17/17 PASS, with updated sections for the new rules (two sockets per PC, a tab shown in both halves and typed into, closing the duplicate keeps the survivor attached, a narrow-window pick changes the shown half) and two new ones: a real mouse drag from a pane title, and the follower's xterm grid equal to the active half's after an inactive-half Chat-to-Terminal switch (failed 23x34 vs 34x34 before the fix).
• Browser demo in headless Chrome: same pane in both halves, independent sidebar picks, Agents-row and half-bar drags, no removed menu items, one half at 390px, equal grids in both halves.
• Two independent adversarial reviews; their blockers (title drag cancelled by pointerdown, follower re-fit on lens/font change, demo geometry) were fixed and re-reviewed: APPROVE.

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📝 Summary

Summary by CodeRabbit

• New Features
  • Split view can now display the same tab in both halves. Selecting items from the sidebar or Agents list opens them in the active half without changing focus.
  • Drag workspace rows, online Agents rows, tabs, pane titles, and half bars to choose which side opens a pane.
  • When both halves show the same tab, the inactive half follows the active half’s pane size; typing still targets the active half.
• Bug Fixes
  • Split view is retained when a pane’s status cannot be confirmed, and closes or replaces a pane only after its closure is confirmed.

The split-view flow now lets each half select independently, including showing the same tab in both halves. It adds pane drag targets from the sidebar, pane titles, and slot bars. In same-tab splits, the inactive terminal follows the active half’s grid. The app, demo transport, regression tests, guide, and changelog reflect these changes.

Changes: Independent Split Halves

Layer / File(s)	Summary
Half selection and docking src/lib/split.ts, src/lib/split.test.ts, src/App.tsx, scripts/split-view-regression.ts	Docking can duplicate a pane or select it without swapping pane positions. Selecting a pane activates its half. Split cleanup checks connected rosters and can replace a closed pane with another pane from its tab.
Shared-tab rendering and terminal sizing src/App.tsx, src/lib/paneSubmitContext.ts, src/components/PaneTerminal.tsx, site/demo/transport.ts, scripts/split-view-regression.ts	Both halves can render the same tab through separate session providers. The inactive half follows the active half’s PTY grid. Demo attachments can preserve the current grid, and geometry updates are broadcast when dimensions change.
Pane drag targets and guidance src/components/AgentSidebar.tsx, src/components/PaneCanvas.tsx, src/App.tsx, docs/guide.md, changelog.d/split-view-independent-halves.md, scripts/split-view-regression.ts	Online Agents rows, pane titles, and split slot bars provide pane drag data. Pane-title drags can dock a pane on a requested side. The guide and changelog describe the split behavior and supported drag targets.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~40 minutes

Change: Feature

Suggested reviewers: yoonwoo-ha, demrom, wouldu-pres

Merge Risk: 🔵 Low · up to 4d490

A stale split half may remain visible after a failed session read until another roster change or manual closure. This is a bounded issue that can be fixed or accepted before merging.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 9 files. (2 skipped: 2…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title clearly and concisely describes the primary change: independent split halves that can display the same tab.
Description check		The description includes the required Change and Validation sections. It explains the behavior, implementation scope, test results, known follow-up, and environment-specific failures. No maintainer-ap…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 9 files. (2 skipped: 2 unsupported.)

✨ Finishing Touches: 💡 1

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Actionable comments posted: 1

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• 🪄 Fix comments: Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @src/App.tsx:
- Around line 927-937: Update the effect containing the `fetchSession` check for
`otherTabRef.current` to include the roster sequence, such as `rosterTick`, in
its dependency list so each roster read retries a failed fetch while preserving
the existing pane-closed check.

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📥 Commits

Reviewing files that changed from the base of the PR and between ece0896 and 4d4906c.

📒 Files selected for processing: 11

• changelog.d/split-view-independent-halves.md
• docs/guide.md
• scripts/split-view-regression.ts
• site/demo/transport.ts
• src/App.tsx
• src/components/AgentSidebar.tsx
• src/components/PaneCanvas.tsx
• src/components/PaneTerminal.tsx
• src/lib/paneSubmitContext.ts
• src/lib/split.test.ts
• src/lib/split.ts

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