bug: two windows on the same chat — terminal size state desyncs across primary flips (stale lastPushed dedupe, dual-primary push race, wrong-cols replay)

[yicheng47]

Description

With two windows showing the same chat tab, the terminal's size state can desync when primary ownership flips between windows. Three mechanisms, from code inspection of the impl 0018/0020 path:

1. Wrong-cols replay on every flip (by design, with a real cost). A secondary window mounts nothing (ChatPaneGroup.tsx renderTerminalPane: secondaryFor(id)?.secondary → null), so a secondary→primary flip is a fresh xterm mount that replays ring bytes recorded at the other window's cols into this window's grid. The live region heals (attach dance → cols push → fix(session): terminal history loss on tab return — ring purge cols-gate + measurable WebGL attach #308 purge → SIGWINCH repaint), but the re-wrapped replay stays as garbled scrollback, accumulating with every flip between differently-sized windows.

2. Size state is frontend-local and never reconciled. Each RunnerTerminal instance dedupes pushes against its own lastPushedColsRef/lastPushedRowsRef (RunnerTerminal.tsx:206), and session_resize is push-only — there is no API to read the PTY's actual size (api.ts:309, commands/session.rs:109). When the other window resizes the PTY, this window's refs go stale: refresh-push-skip (RunnerTerminal.tsx:362-371) can then skip a push the PTY actually needs. Same stale-size class as bug: resuming an old claude-code terminal shows a black screen with cursor at top until a manual window resize #312.

3. Dual-primary push race during flips. Primary is derived from the debounced (80ms), broadcast focus map (windowFocus.ts), so there's an interval during a flip where both windows are mounted and both push conflicting sizes. Last write wins — and the loser can be the window that stays primary, leaving the PTY at the other window's size while nothing on the primary re-pushes (its rect didn't change, so no ResizeObserver event). The agent then repaints at the wrong width into the primary's grid until a manual window resize.

Expected behavior

The primary window's grid and the PTY size converge without a manual resize, and flips between differently-sized windows don't accumulate garbled scrollback.

Suggested direction

An authoritative size read-back — either a session_pty_size query or the current size echoed on resize acks / session events — lets the activation path reconcile against the PTY's real size instead of frontend-local lastPushed refs. That single primitive addresses (2), (3), and the refresh-push-skip half of #312.

Relevant code

• src/components/ChatPaneGroup.tsx — renderTerminalPane secondary gate (unmount/remount on flip).
• src/components/RunnerTerminal.tsx:206,362-371 — lastPushed*Ref dedupe; :1001-1091 replay drain (wrong-cols rewrap).
• src/lib/windowFocus.ts — debounced focus map → isSecondaryFor (dual-primary interval).
• src-tauri/src/commands/session.rs:109 — session_resize (push-only; no size query).

Environment

• OS: macOS
• Version: 0.3.17