fix: instant terminal switching + flicker-free rendering

[aterrylu]

Summary

• VSCode-style session keep-alive: Terminal instances stay mounted across session switches — only CSS visibility toggles. Eliminates the violent flashing caused by destroying and recreating terminals on every switch.
• xterm.js 5 → 6 upgrade: Native DEC 2026 synchronized output support. Claude Code wraps redraws in BSU/ESU sequences — xterm.js 6 buffers these and renders atomically, eliminating the remaining per-update flicker.
• Review fixes: Early-return bug fix (terminals no longer unmount when no session selected), dead code removal, React.memo on SessionPane, lazy WebGL loading, zero-dimension fit guard.

Architecture

graph LR
  A[SessionViewManager] -->|renders all| B[SessionPane A]
  A -->|renders all| C[SessionPane B]
  A -->|renders all| D[SessionPane C]
  B -->|visible| E[useTerminal + WebGL]
  C -->|hidden| F[useTerminal + canvas]
  D -->|hidden| G[useTerminal + canvas]
  style B fill:#4ade80,color:#000
  style C fill:#94a3b8,color:#000
  style D fill:#94a3b8,color:#000

Loading

• All live sessions render simultaneously; only the active one is display: flex
• WebGL addon loads lazily on first visibility (prevents browser context exhaustion)
• FitAddon skips hidden containers (0×0 dimensions) to avoid bogus resize messages
• React.memo on SessionPane prevents re-renders from 5s session poll

Test plan

• Switch between 2+ active sessions — should be instant, no flash
• Create new session while others are running — existing terminals preserved
• Session ends (PTY exit) — removed from list, other sessions unaffected
• Claude Code streaming output — reduced/eliminated flicker vs before
• No session selected — placeholder shown, terminals still alive in background

🤖 Generated with Claude Code

[nox-0x]

🟢 Suggestion

Problem: webglLoaded is a closure-local flag, so once a hidden terminal becomes visible and loads WebGL, that context lives for the lifetime of the terminal instance. With many sessions open and the user having visited each at least once, you can accumulate N WebGL contexts.

Why it matters: Browsers cap WebGL contexts at ~16 (Chrome) or ~8 (Safari). Exceeding this silently falls back to software rendering — which is benign but invisible to the user, and the console.warn here only fires during the initial load attempt, not on the silent eviction.

Suggested fix: Track active WebGL addons and dispose() on hidden terminals, keeping only the visible one GPU-accelerated:

// In useTerminal, after loading WebGL:
const webglAddonRef = useRef<WebglAddon | null>(null);

// When becoming visible (ResizeObserver, offsetWidth > 0):
if (!webglAddonRef.current) {
  try {
    const addon = new WebglAddon();
    terminal.loadAddon(addon);
    webglAddonRef.current = addon;
  } catch (err) { /* fall through to canvas */ }
}

// When becoming hidden (visibility toggle on SessionPane):
// webglAddonRef.current?.dispose(); webglAddonRef.current = null;

Not blocking for the typical 2–5 session use case, but worth addressing before supporting double-digit session counts.

[aterrylu]

Good call — implemented. WebGL addon is now disposed when a terminal becomes hidden (ResizeObserver detects 0×0) and reloaded when it becomes visible again. Only the active terminal holds a GPU context at any time.

[nox-0x]

Clean implementation of the VSCode-style keep-alive pattern — the architecture (SessionViewManager → SessionPane → useTerminal) is well-structured and the guards (zero-dim fit skip, lazy WebGL, isActiveRef for multi-instance status) are all correct. Left one suggestion about WebGL context accumulation across many sessions (not blocking). xterm v6 synchronized output support is a nice bonus that directly addresses the per-update flicker.

[nox-0x]

Previous suggestion (WebGL context accumulation) is fully addressed — disposing on hidden + reloading on visible is the right pattern. One minor nit: sessionId !== null is stricter than the old !sessionId guard; if the store ever initialises sessionId as undefined the placeholder would not render, but in practice this is fine if the type is string | null. Architecture is clean, the ResizeObserver-driven WebGL lifecycle is correct, and the xterm v6 synchronized output support is a solid bonus. Ship it. ✅