feat(split): two panes side by side, by dragging a tab or a sidebar row

[Yoonwoo-Ha]

Change

On a desktop, two panes side by side, as a window snapped to half the screen: drag a tab, or a sidebar row, over the pane area and drop it on the left or right half.

• Dropping. While dragging, both halves are outlined and the one under the pointer fills. Dropped there, the pane opens in that half, beside the pane already open.
• Size. The halves share the area beside the sidebar, or the whole window with the sidebar hidden.
• Each half has:
  • its own tab strip;
  • its own chat or terminal lens, remembered per pane as now;
  • a slim bar with its pane's mark and title, a chat/terminal toggle, and Close this half.
• The active half. The selected pane is the active half's. The header, the palette, the shortcuts and the file viewer act on it, and its bar carries the accent. A press or focus in the other half makes that half active first, so the press then acts there.
• Dropping on an existing split. On the other half: that half takes the pane and becomes active. A half's own pane dropped on the opposite half: the two halves trade sides.
• Kept and cleared. The split is kept across a reload (herdr-web-ui:split). It ends when either pane closes. A window narrower than 769px shows the active half alone. Alerts skip a pane open in either half.

How it is built (src/lib/split.ts, App):

• Two slots. a shows a single pane, and b exists only while split, on bSide. Each slot keeps its pane and its PaneTerminal, so switching the active half or trading sides never attaches a terminal again. Sides are drawn with order.
• Selection. The selected pane stays App's one source of truth. The other half's pane, its side and the active slot are the split state. Picking the other half's pane anywhere (sidebar, palette, alert) makes that half active instead of opening it twice.
• The other half's terminal. It is a second PaneTerminal under its own MachineContext, so a half can be another PC's pane. Its connection callbacks are no-ops, and the header's state stays the active half's.
• Drag data. A tab and a workspace row carry application/x-herdr-pane. The row keeps its reordering data too: dropped on the list it reorders, on the pane area it splits. The area takes the drop in the capture phase, so the composer and the terminal, which take dropped files, never see it.
• Accessibility. Each half's tab strip controls that half's own tab panel: pane-tabpanel for slot a, pane-tabpanel-b for slot b, through a new TabStrip panelId prop.

This is a larger change to how the pane area is laid out, so I'm happy to adjust the interaction or split it up if you see it differently.

Validation

• src/lib/split.test.ts, the split as pure functions:

  • a drop beside a single pane, on either side;
  • a drop onto itself, or with nothing open;
  • a drop on the active half and on the other half;
  • both ways of trading sides;
  • activating the other half;
  • finding the half from the pointer, and reading the drag data.

• New scripts/ui-regression.ts step:

  • a sidebar row is dragged to the right half, and both halves show their own conversation;
  • the dropped half is active and its pane is selected;
  • the halves are equal and side by side;
  • a press makes the left half active without moving either pane, and the split is stored;
  • Close this half leaves one pane.

  It runs after the steps that take the newest WebSocket for the page's own, since the second half opens one. bun run test:ui 84/84, twice.

• By hand, in Chrome at 1440px against a real session:

  • a tab dropped on the right half splits the area (561 / 560px);
  • a press on the left half makes it active;
  • hiding the sidebar widens both halves (721 / 720px);
  • a reload keeps the split;
  • the right half's close leaves one pane;
  • no page errors.

• bun run test:unit 1819 pass, tsc --noEmit clean. The composer-fit regression is unchanged (the same passes as on main).

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📒 Files selected for processing (7)

• CHANGELOG.md
• scripts/split-view-regression.ts
• scripts/ui-regression.ts
• src/App.tsx
• src/components/PaneTerminal.tsx
• src/lib/split.test.ts
• src/lib/split.ts

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• CHANGELOG.md

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📝 Summary

Summary by CodeRabbit

• New Features
  • Added a desktop split view that lets you drag tabs or workspace rows to display two panes side by side.
  • Each pane has its own tabs, chat or terminal view, and controls. Select either pane to make it active, or close one half to return to a single pane.
  • Split layout and active pane persist across reloads. On narrow screens, only the active pane is shown.
• Bug Fixes
  • Alerts for panes visible in the split view are suppressed. When a narrow screen shows only the active pane, alerts for the hidden pane can resume.
• Localization
  • Added Japanese, Korean, and Simplified Chinese translations for split-pane controls and status messages.

Walkthrough

The desktop interface adds a persistent split view with two pane slots. Users can drag panes into either side, switch the active side, and control each pane independently. Narrow windows display only the active pane.

Changes

Desktop split-pane view

Layer / File(s)	Summary
Split state and docking rules src/lib/split.ts, src/lib/split.test.ts	Split-pane helpers determine docking, activation, drop side, and drag-data validity. Split state can be restored from and saved to local storage. Tests cover docking, activation, and invalid drag data.
Pane drag-and-drop and application state src/App.tsx, src/components/Sidebar.tsx, src/components/TabStrip.tsx, src/components/PaneTerminal.tsx	Sidebar rows and pane tabs provide pane drag data. App handles docking, pane selection, split lifecycle, and active-pane state updates. PaneTerminal retains and replays connection, stalled-output, and acknowledged-role state when callbacks change.
Split-pane presentation and regression coverage src/styles.css, src/components/TabStrip.css, src/lib/i18n.*.ts, CHANGELOG.md, scripts/split-view-regression.ts, scripts/ui-regression.ts	Styles and translations support split-pane controls. Browser regression checks cover docking, persistence, alert suppression, focus activation, and independent connection roles.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Feature

Suggested reviewers: devswha

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Merge Risk | ⚪ Minimal · up to f7571

Merge Risk: ⚪ Minimal · up to f7571

The desktop split view adds persistent two-pane behavior. Earlier feedback on roles, stalled output and connection state has been addressed, and no unresolved defects remain from this review. A maintainer-approved CI run is still advisable before merging.

Security Architecture Review

Security architecture risk: 🔵 Low · up to f7571

The split view introduces concurrent access to two panes, but the inspected routing, connection ownership and read-only controls remain intact. No introduced security vulnerability was established. Some uncertainty remains around runtime recovery and cross-machine behavior.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — A split can concurrently expose two pane sessions, including sessions on different machines available through the existing browser identity. Machine IDs select the existing server routing path rather than supplying arbitrary websocket destinations.

Trust Boundaries and Controls

• observed — The inspected server path authenticates websocket requests and checks their origin before upgrade. Watch identities receive read-only connections, and subsequent role requests are clamped to observe. Device revocation closes connections and releases their attachments. These controls predate the PR and are unchanged in the full comparison.

Resilience and Maintainability Implications

• observed — Existing attachment claims contain asynchronous races: detach cancels an outstanding attach, repeated attachment avoids duplicate replay, and queued input rechecks attachment identity and authority before delivery. These mechanisms support failure containment when two slots operate concurrently.

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 47.37% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 19 functions across 11 files. (1 skipped:…	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 main change: adding two side-by-side panes opened by dragging a tab or sidebar row.
Description check		The description includes the required Change and Validation sections. It explains the split-pane behavior, persistence, accessibility, implementation details, regression coverage, manual UI validation…
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.

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Full details: Docstring Coverage

Explanation

Docstring coverage is 47.37% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 19 functions across 11 files. (1 skipped: 1 unsupported.)

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🟡 Minor · Close the split when active-pane fallback runs. · App.tsx:557-573

src/App.tsx:557-573
🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

Close the split when active-pane fallback runs.

When fetchSession confirms that the selected active pane was removed, this effect selects a fallback pane but leaves split unchanged. If the fallback is not split.other, splitOn remains true and the UI continues to show a two-pane split with the fallback and the unrelated split.other pane. Clear the split before selecting the fallback, while preserving the fallback selection.

Suggested fix

-      setSelectedPaneId(fallback(current));
+      setSplit(null);
+      setSelectedPaneId(fallback(current));
       setAutoSelected(true);

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Review comment at @src/App.tsx around lines 557 - 573:
When fetchSession confirms the selected pane was removed, clear the split state
before selecting the fallback in the effect; preserve the fallback selection and
auto-selection behavior.

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Inline comments:
Review comments at @src/App.tsx:
- Around line 1050-1088: Update the active-gated callbacks in the App component
and PaneTerminal activation handling so selecting an already-mounted split half
synchronizes its current connection and role state through onConnectionChange
and onRoleAck, without waiting for a new socket event. Preserve onServerMessage
for newly received messages.

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Outside diff comments:
Review comments at @src/App.tsx:
- Around line 557-573: When fetchSession confirms the selected pane was removed,
clear the split state before selecting the fallback in the effect; preserve the
fallback selection and auto-selection behavior.

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📒 Files selected for processing (12)

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Inline comments:
Review comments at @src/App.tsx:
- Around line 1075-1077: Update the active-pane state flow around
`onServerMessage`, `setRole`, and `applySplit` so activating a pane synchronizes
the header with that terminal’s current role and output-stalled state, rather
than retaining the previous pane’s values or clearing output state. Expose the
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role acknowledgement also resets or derives the role for the newly active pane.

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[devswha]

Shortcut/mobile-policy review of d0d2002 (static review of the active-pane path, not a full-PR approval):

Routing global Mod+Shift actions through the selected/active pane is the right boundary. Keep the existing xterm app-shortcut interception and do not add app actions to the mobile terminal key bar.

Two source-level issues need coverage and fixes before merge:

1. dropIn/chime suppress alerts for splitRef.current.other even when the viewport is narrow and splitOn is false. Split A|B, shrink below the split breakpoint, then let the now-hidden half need input or finish: it is still treated as visible and receives no in-app alert/chime. Suppress for the other half only while it is actually shown, and add a narrow-viewport regression.
2. onFocusCapture activates a half through applySplit, which sets autoSelected(false). That true-to-false transition triggers terminal/composer focusing, so a keyboard user tabbing onto a button in the inactive half can lose that focus. Preserve the element that initiated focus-based activation and test tabbing into the inactive tab strip/composer controls. This path was traced in source, not reproduced in a browser during this review.

GitHub currently reports CONFLICTING/DIRTY, there are no required CI check runs on this head, and the output_stalled/role replay review thread remains unresolved. Refresh against main, resolve that thread, and obtain Fast checks plus Integration and browser on the refreshed head.

Keep this as a separate split-view PR. #480 also changes tab onDragStart; whichever lands second must keep both reorder and split drag payloads.

[Yoonwoo-Ha]

Thanks for the review. All of it is addressed in d6d4e27, on top of main (cc9701c), so the PR is no longer conflicting.

1. Alerts for a hidden half. In-app alerts and the chime now skip the other half's pane only while that half is drawn (splitShownRef, set from splitOn each render). Below the split breakpoint it alerts again.
2. Focus on a Tab into the inactive half. Activation through onFocusCapture now keeps autoSelected (applySplit(…, keepFocus)), so the half's terminal and composer do not take the keyboard from the control the focus went to. A press still activates as before. In the browser, before the change the focus moved off the half's Attach files button when it was tabbed to; now it stays.
3. The open thread. A stalled output is replayed on activation as the connection is. The role case is explained in the thread.

Coverage. A new scripts/split-view-regression.ts runs from ui-regression.ts on a browser context of its own: the other half's socket would otherwise be taken for the main page's own by the steps that read the newest one.

• A split with the other pane on the right, the open one active: blocking the other pane drops no alert.
• At 700px wide (one half shown), blocking it again drops the "Needs input" card.
• Back to two halves, focusing the right half's Attach files makes that half active and leaves the focus on the button.

Each fails without its change (checked by removing the fixes): no card at 700px, then the focus stays on the control it went to.

bun run test:unit 1934 pass, tsc --noEmit clean, bun run test:ui 88/88 twice.

The mobile key bar and the xterm shortcut interception are unchanged. Noted on #480: whichever lands second keeps both the reorder and the split payloads in the tab's onDragStart. The CI checks will need a maintainer's approval to run on this fork head.

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Inline comments:
Review comments at @src/App.tsx:
- Around line 1090-1095: Update App’s role state so each PaneTerminal connection
has its own role value; have onRoleAck update only the role for that connection,
and pass that same value back through the PaneTerminal role prop. Initialize
each connection independently so an observe acknowledgement from one pane cannot
change the other pane’s mode.

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[devswha]

Thanks for the quick turnaround on the 10-08 review. Both of its points are fixed as you describe. I reviewed what changed since that review (d0d2002 → d988e9b: the rebase, d6d4e27 and d988e9b), merged the head locally onto main at v0.4.2 (b82584d), ran the checks below, and drove the split in a real browser on an isolated herdr session. This is a comment, not a request for changes.

Merge state

• It merges with v0.4.2 without a conflict, but the CHANGELOG entry moves with it: v0.4.2 was cut after this branch, and in the merged file your entry sits under ## [0.4.2] → ### Added (line 13), a release that does not contain it. It needs to move under ## [Unreleased].
• "Fast checks" and "Integration and browser" have not run on this head. Only CodeRabbit ran, and every review thread is resolved.

The points of the last review

• Alerts for a hidden half: fixed (splitShownRef). Your regression checks it at 700px.
• Focus on a Tab into the inactive half: fixed (applySplit(…, keepFocus)). Your regression checks it.
• Roles per connection (d988e9b): each half keeps its own role. I read the change and your forged role-ack test covers it.

Findings

High. A finished upload moves the keyboard to the other half (src/components/PaneTerminal.tsx:1031, through src/App.tsx:1065). I reproduced this in a real browser:

1. Split two shell panes, left and right; the left one is active.
2. Drop a file on the left terminal. I held /api/pane/image with page.route so the upload stays in flight.
3. Click the right terminal. The right half is active and has the focus.
4. Let the upload finish. uploadBatch pastes the path into the left terminal and calls term.focus(). The focus lands in the left half, its onFocusCapture calls activateSlot, and the left half becomes active again.

Result: after upload finished: left active=true, focus in left. Whatever the user types next, Enter included, goes to the left pane while they are looking at the right one. Before this PR only one terminal was mounted, and uploadBatch returned early once the user had left the pane (paneRef.current !== pane). With two halves both stay mounted, so that check passes.

Fix: focus the terminal after an upload only when the focus is still inside its own half (or nowhere), and do not let a programmatic focus activate a half; onFocusCapture could require a user-initiated focus. The same applies to term.focus() after a dropped text (PaneTerminal.tsx, the onDrop text branch), and to any delayed restoreFocus.

Medium. Closing the other half moves the surviving pane to a new connection (src/App.tsx:1043, src/App.tsx:719). This is traced in the code, not reproduced. When the right half (slot b) is active and the user closes the left half, or narrows the window below the split width, slot b stops rendering and the active pane is drawn in slot a. Slot b's PaneTerminal unmounts: it detaches its socket and suspends its pending messages (PaneTerminal.tsx:1315). Slot a attaches the same pane on its own socket. A message queued for the agent's next turn was owned by slot b's connection, so it is held for review instead of being sent automatically. That is safe (AGENTS.md: automatic delivery needs the original connection), but the user never left the pane, so it reads as a lost send. The terminal also re-attaches with a fresh replay. Keeping the surviving slot mounted when the split closes would avoid both.

Low. Fixed sleeps in the new regression (scripts/split-view-regression.ts:62, :65, :74, :80, :85, :132). AGENTS.md asks for bounded waits on a condition, never fixed sleeps. The "no alert" checks need a positive signal to wait on (for example, wait until the sidebar shows the blocked state, then for one more status round-trip) rather than 1.2 s.

Bridge invariants

I checked the ones server/AGENTS.md names against this diff; there is no server change:

• Two attaches of one pane: dockPane refuses to put the same pane in both halves (sameTarget, src/lib/split.ts:73, :78, :84). Two different panes from one browser are two sockets on the same server, which is how the server already serves two tabs.
• Resize ownership: each half's terminal is an interact client of its own pane and resizes only that pane, so the two halves never resize the same pane.
• Scrollback: no change to the xterm options; scrollback: 0 holds.
• Roles: per slot, as above.

What I ran

On the merged tree (b82584d + d988e9b):

• bun run check fast (workflow lint, generated types, typecheck, build, unit suite): passed. Output: exit 0.
• bun run check run bun scripts/ui-regression.ts (it runs split-view-regression.ts): 89 PASS lines, 0 failures, exit 0, including your four split checks (alerts only while drawn, Tab into the inactive half, each half's role, drag/press/close).
• The reproduction above, as a Playwright script on an isolated herdr session through bun run check run.
• An adversarial review pass with another agent (Codex) over the diff. Its two findings are the High (which I then reproduced) and the Medium (which I traced).

For the owner to decide

• The High is introduced by this PR and makes a keystroke land in a pane the user is not looking at. In my view it should be fixed before a merge.
• Whether the Medium (a queued send held after closing the other half) should be fixed here or accepted as a known limit.
• #429 (file viewer) moves --chat-w from the terminal stack to document.documentElement, which assumes one PaneTerminal is mounted. With this PR there are two, so the halves would overwrite each other's chat width, and closing one would remove it from both. Whichever of the two lands second has to carry that width differently; I said the same on #429.
• #480 also changes the tab's onDragStart; whichever lands second has to keep both the reorder and the split drag data (as said on 10-08).

[Yoonwoo-Ha]

Thanks for the thorough pass, and for reproducing the High in a real browser. Everything is in f757101, rebased on main at 0.4.3 (13ca352).

Merge state. The changelog entry is under ## [Unreleased] → ### Added again. The diff of CHANGELOG.md against main adds only that entry.

High: a finished upload moves the keyboard. Fixed in two places.

• The terminal. PaneTerminal takes the keyboard by itself, after an upload finishes or after a dropped text, only when the focus is nowhere or already in its own half (mayTakeFocus: the focused element's .pane-slot must contain this terminal's host). Single-pane behaviour is unchanged.

• Activation by focus. A half becomes active through onFocusCapture only right after a key (lastKeyAtRef, KEY_FOCUS_MS = 1 s, recorded on a capturing keydown). A Tab still activates a half. A focus moved by the app's own code no longer does.

• Regression. Your reproduction is now a check in split-view-regression.ts:

  1. Split two panes in the terminal lens, with the left one active.
  2. Drop a file on the left terminal while /api/pane/image is held.
  3. Click the right terminal.
  4. Release the upload. The right half must stay active, and the focus must stay inside it.

  Before the change it fails with the right half stays active.

Medium: closing the other half moves the surviving pane to a new connection. Fixed.

• A single pane is now drawn in a solo slot. The half left when a split ends keeps its slot, and with it its PaneTerminal, socket and pending messages.
• A window too narrow for both draws the split's active slot, not slot a.
• A later drop opens the new pane in the other slot (dockPane(…, solo), with a unit case for a solo slot b). The closed half's role starts over at interact.
• The single pane's tab panel keeps the page's pane-tabpanel id whichever slot draws it.
• Regression. With the right half active, closing the left half must close the left half's socket and leave the right half's open. Before the change it fails with the active half keeps its connection.

Low: fixed sleeps. None are left in split-view-regression.ts; every wait is on a condition.

• "No alert" checks. After the blocked state shows in the sidebar, a third pane reports working. The app shows that status only after it has handled the block, so no card by then means none.
• Returning to idle waits for the blocked state to leave the row.
• Effects of a render are covered by waiting for two animation frames.
• Focus check. The key and the focus now come in one task (a dispatched keydown, then focus()), so the 1 s window holds on a slow runner. A real Tab from the bar landed on the composer's resize handle, which would not have caught a composer taking the focus.
• Role check. It reads a per-half data-role from the render that shows the right half watching, instead of waiting a while.

Runs.

• bun run test:unit: 2016 pass. tsc --noEmit: clean.
• split-view-regression.ts on its own: 3 of 3, all five checks.
• Full bun scripts/ui-regression.ts, six runs on this head:
  • five passed with 99 PASS lines;
  • one stopped after 71, in the existing queued-messages step (legacy held rows restored, the Stop agent button count after a reload). That step runs after the split checks, on the main page, and did not recur in the three runs after it. main passed 3 of 3 in the same session.
• An earlier full run stopped in terminal-command-arrows-regression.ts (readline fixture must start) while the machine's load average was about 9. That was before any split check runs.

Not done. "Fast checks" and "Integration and browser" still need your approval to run on this fork head.

Noted.

• On feat(viewer): Markdown preview and highlighted code with line numbers #429: --chat-w set on document.documentElement assumes one mounted PaneTerminal. Whichever lands second carries the width per terminal, or per half.
• On feat(tabs): reorder tabs from the tab strip #480: whichever lands second keeps both the reorder and the split drag payloads in the tab's onDragStart.

[flam-ing]

@devswha 데스크톱 웹 UI에서 여러 에이전트를 나란히 보고 싶은 사용자의 피드백입니다. 현재 main은 pane을 하나씩 보여주지만, 이 PR에 드래그 분할과 드롭 전 좌우 미리보기가 이미 포함되어 있는 것을 확인했습니다.

이 PR이나 #657의 분할 UI가 반영될 때, 아래처럼 기능을 쉽게 발견할 수 있으면 좋겠습니다.

• 사이드바 행이나 탭을 조금 끌어 드래그가 시작되면, 화면 가장자리를 정확히 찾아가기 전에도 분할할 수 있다는 안내나 드롭 영역이 나타나기.
• pane 영역 위에 드래그한 채 호버하면 어느 쪽에 놓일지 미리 보이고, 놓았을 때만 분할 확정. 이미 구현하신 좌우 미리보기 동작은 유지됐으면 합니다.
• 기존 사이드바 재정렬과 충돌한다면 Option(macOS)/Alt + 드래그를 분할용 보조 제스처로 제공하는 것도 좋겠습니다.
• Esc 또는 영역 밖으로 나가 취소하면 기존 배치와 입력 포커스를 유지하기. 단순 마우스 호버만으로 세션이 전환되거나 분할되길 바라는 것은 아닙니다.

새 분할 기능을 중복 요청하려는 것은 아니고, 진행 중인 구현에 드래그 시작 시점의 안내와 호버 사용성을 함께 고려해 달라는 요청입니다. 감사합니다.

---

@Yoonwoo-Ha 말씀드린 부분을 기존 구현 위에서 조금만 더 다듬어 보고 싶어, 이 PR의 feat/split-view 브랜치를 대상으로 작은 후속 패치를 만들어 보았습니다: Yoonwoo-Ha/herdr-web-ui#1.

분할 동작은 그대로 두고, 드래그를 시작하면 좌우 안내가 먼저 나타나고 미리보기가 120ms로 부드럽게 이동하도록 했습니다. 가운데에서 손이 조금 흔들릴 때 좌우가 계속 바뀌지 않도록 하고, 실제 드롭도 보이는 쪽으로 맞췄습니다. 기존 PR에 이 정도 사용성 개선을 함께 얹어 보면 어떨까요? 방향이 맞는 부분만 편하게 골라 반영하셔도 좋겠습니다.

기본 검사는 1,996개 통과 / 33개 생략 / 실패 0개이며, 가상 데모에서 실제 마우스 드래그와 경계·취소 동작도 확인했습니다. 화면 캡처와 실행하지 못한 검증 범위는 후속 PR에 적었습니다. 감사합니다.

[Yoonwoo-Ha]

Rebased onto main at 0.4.5 (6005735); the head is now 38ba9e5.

Two conflicts, both resolved to keep main's change:

• PaneTerminal's new props. It gained title and backgroundWait. Each half now passes its own pane's values (displayPaneTitle(pane) and pane.background_wait).
• The sidebar row's drag. It keeps main's coarse-pointer guard on draggable and still carries the pane for a split.

The changelog entry stays under ## [Unreleased] → ### Added.

Checks on this head:

• bun run test:unit: 2294 pass. Typecheck, tools typecheck included: clean.
• split-view-regression.ts: all five checks pass.
• Full bun scripts/ui-regression.ts: 112 PASS lines, twice.

[devswha]

Thanks for this, and for the rebases and careful answers through several reviews. Closing it for now, not because of a defect:

• main has since moved to drawing herdr's own split layout (feat(panes): split, zoom, swap, resize and clear panes from the web #726 split/zoom/swap/resize/clear, feat(ui): align pane controls and navigation with herdr TUI #747 PaneCanvas), and a second, browser-side split path next to it needs a product decision first.
• feat(layout): arrange multiple pane views with docking presets #657 proposed a broader version of the same idea (a grid with presets).

I've gathered both proposals, @flam-ing's discoverability feedback and the open questions in #792, with a summary of how this PR worked. If the direction is agreed there, a fresh PR on current main would be very welcome, and this branch is a good starting point.