Xterm JS

xterm.js Best Practices

Overview

This skill provides comprehensive best practices for building terminal applications with xterm.js, React, and WebSockets. It captures critical patterns discovered through debugging production terminal applications, including state management, WebSocket communication, React hooks integration, and terminal lifecycle management.

When to Use This Skill

Use this skill when:

• Building or debugging xterm.js terminal implementations
• Integrating xterm.js with React (hooks, state, refs)
• Implementing WebSocket-based terminal I/O
• Managing terminal persistence with tmux or similar backends
• Refactoring terminal-related React components into custom hooks
• Debugging terminal initialization, resize, or rendering issues
• Implementing split terminal layouts or multi-window terminal management
• Working on detach/reattach terminal functionality

Core Best Practices

1. Refs and State Management

Critical Pattern: Clear Refs When State Changes

Refs persist across state changes. When clearing state, also clear related refs.

// CORRECT - Clear both state AND ref
if (terminal.agentId) {
  clearProcessedAgentId(terminal.agentId)  // Clear ref
}
updateTerminal(id, { agentId: undefined })  // Clear state

Key Insight:

• State (Zustand/Redux) = what the terminal is
• Refs (useRef) = what we've processed
• When state changes, check if related refs need updating

Common Scenario: Detach/reattach workflows where the same agentId returns from backend. Without clearing the ref, the frontend thinks it already processed this agentId and ignores reconnection messages.

See references/refs-state-patterns.md for detailed examples.

2. WebSocket Message Types

Critical Pattern: Know Your Destructive Operations

Backend WebSocket handlers often have different semantics for similar-looking message types:

• type: 'disconnect' - Graceful disconnect, keep session alive
• type: 'close' - FORCE CLOSE and KILL session (destructive!)

// WRONG - This KILLS the tmux session!
wsRef.current.send(JSON.stringify({
  type: 'close',
  terminalId: terminal.agentId,
}))

// CORRECT - For detach, use API endpoint only
await fetch(`/api/tmux/detach/${sessionName}`, { method: 'POST' })
// Don't send WebSocket message - let PTY disconnect naturally

Key Insight: Read backend code to understand what each message type does. "Close" often means "destroy" in WebSocket contexts.

See references/websocket-patterns.md for backend routing patterns.

3. React Hooks & Refactoring

Critical Pattern: Identify Shared Refs Before Extracting Hooks

When extracting custom hooks that manage shared resources:

// WRONG - Hook creates its own ref
export function useWebSocketManager(...) {
  const wsRef = useRef<WebSocket | null>(null)  // Creates NEW ref!
}

// RIGHT - Hook uses shared ref from parent
export function useWebSocketManager(
  wsRef: React.MutableRefObject<WebSocket | null>,  // Pass as parameter
  ...
) {
  // Uses parent's ref - all components share same WebSocket
}

Checklist Before Extracting Hooks:

• Map out all refs (diagram which components use which refs)
• Check if ref is used outside the hook
• If ref is shared → pass as parameter, don't create internally
• Test with real usage immediately after extraction

See references/react-hooks-patterns.md for refactoring workflows.

4. Terminal Initialization

Critical Pattern: xterm.js Requires Non-Zero Container Dimensions

xterm.js cannot initialize on containers with 0x0 dimensions. Use visibility-based hiding, not display:none.

// WRONG - Prevents xterm initialization
<div style={{ display: isActive ? 'block' : 'none' }}>
  <Terminal />
</div>

// CORRECT - All terminals get dimensions, use visibility to hide
<div style={{
  position: 'absolute',
  top: 0, left: 0, right: 0, bottom: 0,
  visibility: isActive ? 'visible' : 'hidden',
  zIndex: isActive ? 1 : 0,
}}>
  <Terminal />
</div>

Why This Works:

• All terminals render with full dimensions (stacked via absolute positioning)
• xterm.js can initialize properly on all terminals
• visibility: hidden hides inactive terminals without removing dimensions
• Use isSelected prop to trigger refresh when tab becomes active

Common Scenario: Tab-based terminal UI where switching tabs should show different terminals. After refresh, only active tab would render if using display: none.

5. useEffect Dependencies for Initialization

Critical Pattern: Early Returns Need Corresponding Dependencies

If a useEffect checks a ref and returns early, include ref.current in dependencies so it re-runs when ref becomes available.

// WRONG - Only runs once, may return early forever
useEffect(() => {
  if (!terminalRef.current) return  // Returns if null
  // Setup ResizeObserver
}, [])  // Never re-runs!

// CORRECT - Re-runs when ref becomes available
useEffect(() => {
  if (!terminalRef.current) return
  // Setup ResizeObserver
}, [terminalRef.current])  // Re-runs when ref changes!

Common Pattern: Wait for DOM refs AND library instances (xterm, fitAddon) before setup:

useEffect(() => {
  if (!terminalRef.current?.parentElement ||
      !xtermRef.current ||
      !fitAddonRef.current) {
    return  // Wait for all refs
  }
  // Setup ResizeObserver
}, [terminalRef.current, xtermRef.current, fitAddonRef.current])

6. Session Naming & Reconnection

Critical Pattern: Use Consistent Session Identifiers

When reconnecting, use the existing sessionName to find the existing PTY. Don't generate a new one.

// CORRECT - Reconnect to existing session
const config = {
  sessionName: terminal.sessionName,  // Use existing!
  resumable: true,
  useTmux: true,
}

// WRONG - Would create new session
const config = {
  sessionName: generateNewSessionName(),  // DON'T DO THIS
}

Key Insight: Tmux sessions have stable names. Use them as the source of truth for reconnection.

7. Multi-Window Terminal Management

Critical Pattern: Backend Output Routing Must Use Ownership Tracking

For multi-window setups, track which WebSocket connections own which terminals. Never broadcast terminal output to all clients.

// Backend: Track ownership
const terminalOwners = new Map()  // terminalId -> Set<WebSocket>

// On output: send ONLY to owners (no broadcast!)
terminalRegistry.on('output', (terminalId, data) => {
  const owners = terminalOwners.get(terminalId)
  owners.forEach(client => client.send(message))
})

Why: Broadcasting terminal output causes escape sequence corruption (DSR sequences) in wrong windows.

Frontend Pattern: Filter terminals by windowId before adding to agents:

// Check windowId BEFORE adding to webSocketAgents
if (existingTerminal) {
  const terminalWindow = existingTerminal.windowId || 'main'
  if (terminalWindow !== currentWindowId) {
    return  // Ignore terminals from other windows
  }
  // Now safe to add to webSocketAgents
}

See CLAUDE.md "Multi-Window Support - Critical Architecture" section for complete flow.

8. Testing Workflows

Critical Pattern: Test Real Usage Immediately After Refactoring

TypeScript compilation ≠ working code. Always test with real usage:

# After refactoring:
npm run build              # 1. Check TypeScript
# Open http://localhost:5173
# Spawn terminal            # 2. Test spawning
# Type in terminal          # 3. Test input (WebSocket)
# Resize window             # 4. Test resize (ResizeObserver)
# Spawn TUI tool            # 5. Test complex interactions

Refactoring Checklist:

• TypeScript compilation succeeds
• Spawn a terminal (test spawning logic)
• Type in terminal (test WebSocket communication)
• Resize window (test ResizeObserver)
• Spawn TUI tool like htop (test complex ANSI sequences)
• Check browser console for errors
• Check backend logs
• Run test suite: npm test

Prevention: Don't batch multiple hook extractions. Extract one, test, commit.

9. Debugging Patterns

Critical Pattern: Add Diagnostic Logging Before Fixing

When debugging complex state issues, add comprehensive logging first to understand the problem:

// BEFORE fixing, add logging:
console.log('[useWebSocketManager] 📨 Received terminal-spawned:', {
  agentId: message.data.id,
  requestId: message.requestId,
  sessionName: message.data.sessionName,
  pendingSpawnsSize: pendingSpawns.current.size
})

// Log each fallback attempt:
if (!existingTerminal) {
  existingTerminal = storedTerminals.find(t => t.requestId === message.requestId)
  console.log('[useWebSocketManager] 🔍 Checking by requestId:',
    existingTerminal ? 'FOUND' : 'NOT FOUND')
}

Benefits:

• Shows exactly which code path is executing
• Reveals data mismatches (wrong ID, missing state, etc.)
• Helps users self-diagnose issues
• Can be left in for production debugging

10. Multi-Step State Changes

Critical Pattern: Handle All Side Effects When Changing State

When a state change affects multiple systems, update all of them.

Checklist for Terminal State Changes:

• Update Zustand state (terminal properties)
• Clear/update refs (processedAgentIds, pending spawns)
• Notify WebSocket (if needed)
• Clean up event listeners
• Update localStorage (if using persist)

Example (Detach):

// 1. API call
await fetch(`/api/tmux/detach/${sessionName}`, { method: 'POST' })

// 2. Clear ref (DON'T FORGET THIS!)
if (terminal.agentId) {
  clearProcessedAgentId(terminal.agentId)
}

// 3. Update state
updateTerminal(id, {
  status: 'detached',
  agentId: undefined,
})

11. Tmux Split Terminals & EOL Conversion

Critical Pattern: Disable EOL Conversion for Tmux Sessions

When multiple xterm.js instances share a tmux session (e.g., React split terminals), enabling convertEol: true causes output corruption.

Problem:

• Tmux sends terminal sequences with proper line endings (\n)
• xterm with convertEol: true converts \n → \r\n independently
• Each xterm instance converts the SAME tmux output differently
• Result: text bleeding between panes, misaligned split divider

Solution:

const isTmuxSession = !!agent.sessionName || shouldUseTmux;

const xtermOptions = {
  theme: theme.xterm,
  fontSize: savedFontSize,
  cursorBlink: true,
  scrollback: isTmuxSession ? 0 : 10000,

  // CRITICAL: Disable EOL conversion for tmux
  convertEol: !isTmuxSession,  // Only convert for regular shells
  windowsMode: false,          // Ensure UNIX-style line endings
};

Why This Works:

• Tmux sessions: convertEol: false → xterm displays raw PTY output
• Regular shells: convertEol: true → xterm converts for Windows compatibility
• Both xterm instances handle tmux output identically → no corruption

Key Insight: Tmux is a terminal multiplexer that manages its own terminal protocol. Multiple xterm instances sharing one tmux session must handle output identically to prevent corruption.

Reference: Tmux EOL Fix Gist - Complete guide with font normalization patterns

12. Resize & Output Coordination

Critical Pattern: Don't Resize During Active Output

Resizing terminals (especially tmux) sends SIGWINCH which triggers a full screen redraw. During active output streaming, this causes "redraw storms" where the same content appears multiple times.

// Track output timing
const lastOutputTimeRef = useRef(0)
const OUTPUT_QUIET_PERIOD = 500  // Wait 500ms after last output

// In output handler
const handleOutput = (data: string) => {
  lastOutputTimeRef.current = Date.now()
  xterm.write(data)
}

// Before any resize operation
const safeToResize = () => {
  const timeSinceOutput = Date.now() - lastOutputTimeRef.current
  return timeSinceOutput >= OUTPUT_QUIET_PERIOD
}

Critical Pattern: Two-Step Resize Trick for Tmux

Tmux sometimes doesn't properly rewrap text after dimension changes. The "resize trick" forces a full redraw:

const triggerResizeTrick = () => {
  if (!xtermRef.current || !fitAddonRef.current) return

  const currentCols = xtermRef.current.cols
  const currentRows = xtermRef.current.rows

  // Step 1: Resize down by 1 ROW (sends SIGWINCH)
  // CRITICAL: Use rows, NOT columns! Column changes can cause tmux status bar
  // to wrap when sidebar is narrow, corrupting the terminal display.
  // Row changes trigger the same SIGWINCH without affecting horizontal layout.
  const minRows = Math.max(1, currentRows - 1)
  xtermRef.current.resize(currentCols, minRows)
  sendResize(currentCols, minRows)

  // Step 2: Fit to container (sends another SIGWINCH)
  setTimeout(() => {
    fitAddonRef.current.fit()
    const finalCols = xtermRef.current.cols
    const finalRows = xtermRef.current.rows
    sendResize(finalCols, finalRows)
  }, 200)
}

Why rows instead of columns? Shrinking columns by 1 can cause the tmux status bar to wrap when the sidebar is already narrow (e.g., Chrome bookmarks bar open). This single-character wrap corrupts terminal display. Row changes avoid this because status bar width is column-bound, not row-bound.

Critical Pattern: Clear Write Queue After Resize Trick

The two-step resize causes TWO tmux redraws. If you're queueing writes during resize, you'll have duplicate content:

const writeQueueRef = useRef<string[]>([])
const isResizingRef = useRef(false)

// During resize trick
isResizingRef.current = true
// ... do resize ...
isResizingRef.current = false

// CRITICAL: Clear queue instead of flushing after resize trick
// Both redraws are queued - flushing writes duplicate content!
writeQueueRef.current = []

Critical Pattern: Output Guard on Reconnection

When reconnecting to an active tmux session (e.g., page refresh during Claude streaming), buffer initial output to prevent escape sequence corruption:

const isOutputGuardedRef = useRef(true)
const outputGuardBufferRef = useRef<string[]>([])

// Buffer output during guard period
const handleOutput = (data: string) => {
  if (isOutputGuardedRef.current) {
    outputGuardBufferRef.current.push(data)
    return
  }
  xterm.write(data)
}

// Lift guard after initialization (1000ms), flush buffer, then force resize
useEffect(() => {
  const timer = setTimeout(() => {
    isOutputGuardedRef.current = false

    // Flush buffered output
    if (outputGuardBufferRef.current.length > 0) {
      const buffered = outputGuardBufferRef.current.join('')
      outputGuardBufferRef.current = []
      xtermRef.current?.write(buffered)
    }

    // Force resize trick to fix any tmux state issues
    setTimeout(() => triggerResizeTrick(true), 100)
  }, 1000)

  return () => clearTimeout(timer)
}, [])

Critical Pattern: Track and Cancel Deferred Operations

Multiple resize events in quick succession create orphaned timeouts. Track them:

const deferredResizeTrickRef = useRef<NodeJS.Timeout | null>(null)
const deferredFitTerminalRef = useRef<NodeJS.Timeout | null>(null)

// On new resize event, cancel pending deferred operations
const handleResize = () => {
  if (deferredResizeTrickRef.current) {
    clearTimeout(deferredResizeTrickRef.current)
    deferredResizeTrickRef.current = null
  }
  if (deferredFitTerminalRef.current) {
    clearTimeout(deferredFitTerminalRef.current)
    deferredFitTerminalRef.current = null
  }

  // Schedule new operation
  deferredFitTerminalRef.current = setTimeout(() => {
    deferredFitTerminalRef.current = null
    fitTerminal()
  }, 150)
}

See references/resize-patterns.md for complete resize coordination patterns.

13. Tmux-Specific Resize Strategy

Critical Pattern: Skip ResizeObserver for Tmux Sessions

Tmux manages its own pane dimensions. ResizeObserver firing on container changes (focus, clicks, layout) causes unnecessary SIGWINCH signals:

useEffect(() => {
  // For tmux sessions, only send initial resize - skip ResizeObserver
  if (useTmux) {
    console.log('[Resize] Skipping ResizeObserver (tmux session)')
    return  // Don't set up observer at all
  }

  // For regular shells, use ResizeObserver
  const resizeObserver = new ResizeObserver((entries) => {
    // ... handle resize
  })

  resizeObserver.observe(containerRef.current)
  return () => resizeObserver.disconnect()
}, [useTmux])

Why Tmux Is Different:

• Regular shells: Each xterm instance owns its PTY, resize freely
• Tmux sessions: Single PTY with tmux managing internal panes
• Tmux receives SIGWINCH and redraws ALL panes
• Multiple resize events = multiple full redraws = corruption

For Tmux:

• DO resize: Once on initial connection (sets viewport)
• DO resize: On actual browser window resize
• DON'T resize: On focus, tab switch, container changes

Resources

references/

This skill includes detailed reference documentation organized by topic:

• refs-state-patterns.md - Ref management patterns and examples
• websocket-patterns.md - WebSocket communication and backend routing
• react-hooks-patterns.md - React hooks refactoring workflows
• testing-checklist.md - Comprehensive testing workflows
• split-terminal-patterns.md - Split terminal and detach/reattach patterns
• advanced-patterns.md - Advanced patterns (emoji width fix, mouse coordinate transformation, tmux reconnection)
• resize-patterns.md - Resize coordination and output handling

Load these references as needed when working on specific aspects of terminal development.

Highlights from advanced-patterns.md:

• Unicode11 Addon - Fix emoji/Unicode width issues (2 days of debugging → 1 line fix)
• Mouse Coordinate Transformation - Handle CSS zoom/transform on terminal containers
• Tmux Reconnection Best Practices - Prevent reconnecting to wrong sessions

scripts/

No scripts included - xterm.js integration is primarily about patterns and architecture, not executable utilities.

assets/

No assets included - this skill focuses on best practices and patterns rather than templates.

Implementation Principles

Avoid over-engineering. Only make changes directly requested or clearly necessary. Keep solutions simple and focused. Do not add features, refactor code, or make improvements beyond what was asked.

Read and understand relevant files before proposing code edits. Do not speculate about code you have not inspected.