Codeman/docs/split-pane-sessions-plan.md

# Split-Pane Sessions — Design Spec

**Status**: Implemented (v1)
**Author**: Claude (session with Tim), 2026-09-15
**Scope**: v1 only. v2 items are named and explicitly deferred, not designed.

## Problem

Codeman's terminal area shows exactly one active session (pane) at a time —
switching panes re-binds the single xterm instance and the single WebSocket
to a different session. Multi-monitor spanning (`scripts/span-codeman.sh` /
`span-codeman.ps1`) turned out to solve a different problem: it makes one
browser window bigger, but that window still shows one session; floating
subagent windows are draggable overlays on top of it, not tiled panes. There
is no way today to see two live sessions (e.g. `w1-codeman` and
`w1-mcp-memory`) side-by-side in one window, even on a monitor wide enough to
fit both.

## Goal (v1)

From the active session, open a **second, independent, fully live session**
in a pane beside it — draggable divider, side-by-side only. Closing the
second pane collapses back to today's normal single-pane view. No
persistence: a page reload always returns to single-pane. Floating
subagent/Ultracode windows keep their current behavior unchanged (global,
unconstrained across the whole viewport, split or not).

Explicitly out of scope for v1 (v2 candidates, not designed here):
- More than 2 panes / grid layouts
- Vertical (stacked) splits
- Drag-a-tab-to-split as a trigger (v1 trigger is an explicit button + picker)
- Persisting the split layout across reload or across devices
- Mobile/tablet layouts (viewport is too narrow for this to make sense; gated
  to desktop widths the same way `home-sessions.js`'s rail is)
- Feature parity between the two panes (see "Pane B is deliberately plainer"
  below)

## Current architecture (why this isn't a CSS change)

`terminal-ui.js` is built entirely around **singleton** state: `this.terminal`
(one xterm instance), `this._ws`/`this._wsSessionId` (one WebSocket, rebound
on every pane switch via `_disconnectWs()` + `_connectWs(newId)`), a
`this._xtermSnapshots` map used only to restore scrollback into that one
terminal when switching back to a session. Roughly 280 references to this
singleton state exist across the file (input handling, resize/fit, sizing-
token claims, mobile touch gestures, CJK IME, local-echo overlay wiring,
keyboard accessory bar, link providers, etc.).

Showing two sessions at once therefore requires a second, independently
alive xterm + WebSocket pair running concurrently — not a layout change to
one shared instance.

**Related prior art**: `detachSession(id)` (app.js) already opens one session
in a genuinely separate browser window (`isSoloWindow` mode) with its own
independent WebSocket, and two of those can already be snapped side-by-side
today with zero new code. That covers "two sessions visible at once" but not
what this spec is for: one Codeman window with two panes and a divider you
can drag without leaving your seat, each still a full participant in that
window's floating subagent windows, header, and settings. This spec builds
past detach, not a duplicate of it.

**Server-side check (done, not just assumed)**: `MAX_WS_PER_SESSION = 5`
(`src/web/routes/ws-routes.ts`), scoped by `clientId:tabNonce`
(`ws-connection-registry.ts`). Splitting always opens a *different* session
in the second pane (self-splitting is disallowed, see below), so this is two
sessions each getting their normal one connection — the existing cap is
irrelevant here and needs no server change.

## Key design decision: Pane B is deliberately plainer than Pane A

Porting all ~280 singleton behaviors to a second, symmetric pane is not
worth it for v1 — most of that code is input-quality-of-life for **mobile/
touch** (local-echo overlay, CJK IME textarea, touch gesture handling,
keyboard accessory bar), and this feature is desktop-only by nature (a split
view needs a wide viewport). So:

- **Pane A** (the session that was already active when you opened the split)
  stays exactly what it is today — `this.terminal`, `this._ws`, unchanged
  code path, zero regression risk.
- **Pane B** is a new, smaller `SplitTerminalPane` object: its own xterm
  instance + fit addon, its own WebSocket to `/ws/sessions/:id/terminal`,
  resize-on-divider-drag, and plain keyboard input. It does **not** get the
  local-echo overlay, CJK IME composition, touch/mobile handlers, or the
  keyboard accessory bar. On a desktop, typing directly into an xterm
  instance with no overlay is exactly how Codeman behaved before the local-
  echo overlay existed for touch devices — normal, not degraded, for a
  keyboard-and-mouse user.

If this asymmetry actually bothers you in daily use, promoting Pane B to full
parity is a scoped v2 (extract the shared logic already once you have two
call sites to compare, rather than guessing the right abstraction now).

One more asymmetry worth naming here rather than discovering by surprise:
while both panes accept keyboard input, the global capture-phase shortcut
handler (`app.js`) always resolves against Pane A — it has no notion of
which pane currently has focus. So Ctrl+L or Ctrl+W typed while Pane B has
focus clears or closes Pane A, not the session you were actually typing
into. Not fixed for v1, same reasoning as the rest of this section.

## Components

### 1. `SplitTerminalPane` (new, `terminal-split.js`)

A small class, one instance per secondary pane:
- `constructor(sessionId, mountEl)`
- `connect()` — creates the xterm instance (same theme/font config as the
  primary, read from the same settings so it doesn't visually clash), opens
  `/ws/sessions/:id/terminal`, wires input → WS, WS → terminal write
- `fit()` — calls the fit addon; called on divider drag (rAF-throttled) and
  on window resize
- `destroy()` — disposes the xterm instance, closes the WS cleanly

No snapshot/scrollback-restore map is needed the way `_xtermSnapshots` exists
for Pane A — Pane B is destroyed on close, not hidden-and-restored, since
there's no persistence requirement.

### 2. Split container (layout)

```
.terminal-split-container         (flex row, only rendered when split is active)
├── .terminal-wrap                (existing element, Pane A — untouched)
├── .split-divider                (new, draggable seam)
└── .terminal-pane-b              (new, hosts SplitTerminalPane's xterm + a
                                    small header: session name + × close button)
```

When not split, `.terminal-wrap` renders exactly as it does today (no
wrapping container at all, to keep the no-split path byte-identical to
current behavior). Splitting inserts the container and reparents
`.terminal-wrap` into it as the first child — same reparenting pattern
already used by `applySessionListLayout()` for `#sessionTabs`, so this isn't
a new pattern for the codebase.

Default split is 50/50 (`flex-basis: 50%` each). Divider drag updates both
panes' `flex-basis` live (rAF-throttled) and calls `fit()` on **both**
terminals per tick, clamped to 20%/80% so neither pane can be dragged into an
unusably thin sliver.

### 3. Trigger UI

A **"Split"** button (header, opt-in like the other header buttons —
`showSplitButton`, default off, same pattern as `showMultiMonitorButton`)
opens a small picker listing your other open sessions (reuses
`this.sessions`/`sessionOrder`, filtered to exclude the currently active
session — you cannot split a session against itself). Picking one:
1. Creates the split container, reparents `.terminal-wrap`
2. Instantiates `SplitTerminalPane` for the chosen session in `.terminal-pane-b`
3. Button state flips to "close split" (or Pane B's own header × does it)

Closing (via Pane B's × or the header button toggling off):
1. `SplitTerminalPane.destroy()`
2. Removes `.terminal-split-container`, reparents `.terminal-wrap` back to
   its original location at 100% width
3. Fires a resize/fit on Pane A (same `ResizeObserver`-driven fit already in
   place today — no new code needed here, it fires naturally once the
   container's size changes)

v2 note (not designed): dragging a session tab onto the active pane as an
alternate trigger. You confirmed right-click doesn't work today (Codeman
doesn't intercept it) and declined a keybind, so v1 is button+picker only.

### 4. Failure / edge cases

- **The Pane B session ends or is deleted while split is active** → treat
  identically to the user closing Pane B manually: destroy the pane, collapse
  to Pane A at full width.
- **The Pane A session ends while split is active** → Pane B is promoted:
  it becomes the new single full-width pane (reusing today's normal
  single-pane code path means Pane B's `SplitTerminalPane` must hand off to
  a real `this.terminal`/`this._ws` binding — simplest correct approach is
  to just collapse the split and let normal session-select logic reopen
  Pane B's session as the new primary, rather than trying to promote the
  lightweight pane object in place).
- **Both end** → falls through to today's normal "no active session" /
  welcome-screen state.
- **Subagent/Ultracode floating windows** → no design work needed; they're
  already positioned independent of `.terminal-wrap`'s layout, so they
  continue to float over whichever pane(s) are on screen, unconstrained,
  exactly as today.

## Testing

- Unit: `SplitTerminalPane` connect/fit/destroy lifecycle (mock WS, like
  existing terminal tests use `TEST_PTY_SCRIPT`).
- Route/integration: opening two WS connections to two different sessions
  from one simulated client concurrently — confirms the existing per-session
  cap and connection registry need no changes.
- Browser (Playwright, `test/browser` since this is desktop-viewport-gated
  UI): open split via button+picker, verify both panes render live output
  independently, drag divider and confirm both refit, close Pane B and
  confirm Pane A returns to full width, kill the Pane B session externally
  and confirm auto-collapse.

## Open questions for review

None blocking — the scope-narrowing decisions above (Pane B feature parity,
no persistence, side-by-side only, button+picker trigger) came directly from
your answers during brainstorming. Flag anything here you want reconsidered.