fix key repeat: only create new state when previous is acked (match Dart client)

The Dart mosh client only calls sendNew() when hasPendingState is false.
The Go client was calling SetPending on every tick with dirty=true, which
reset diffSent and caused new sequence numbers for the same cumulative
payload. Now tick() checks AckedByRemote >= SentNum before calling
SetPending — at most one unacked state in flight, matching the Dart
client's behavior.

client.go

@@ -12,7 +12,7 @@ import (
 // Client is a mosh client. It connects to a mosh server over UDP,
 // handles the SSP transport, and provides send/recv for terminal I/O.
 type Client struct {
-	conn      *net.UDPConn
+	conn      Conn
 	transport *Transport
 	ocb       *OCB
 
@@ -32,8 +32,8 @@ type Client struct {
 	wg   sync.WaitGroup
 }
 
-// Dial connects to a mosh server. The key is the base64-encoded mosh key
+// Dial connects to a mosh server over UDP. The key is the base64-encoded
-// (with or without padding).
+// mosh key (with or without padding).
 func Dial(host string, port int, key string) (*Client, error) {
 	// Pad key for base64 if needed.
 	for len(key)%4 != 0 {
@@ -57,6 +57,12 @@ func Dial(host string, port int, key string) (*Client, error) {
 		return nil, err
 	}
 
+	return DialConn(conn, ocb)
+}
+
+// DialConn creates a mosh client over an existing datagram connection.
+// Use this with WebTransport or other non-UDP transports.
+func DialConn(conn Conn, ocb *OCB) (*Client, error) {
 	c := &Client{
 		conn:             conn,
 		transport:        NewTransport(ocb, false),
@@ -77,6 +83,29 @@ func Dial(host string, port int, key string) (*Client, error) {
 	return c, nil
 }
 
+// DialConnManual creates a mosh client without an internal sendLoop.
+// The caller must call Tick() periodically (e.g., every 8-16ms) to flush
+// outgoing datagrams. Use this in WASM where Go timers are unreliable.
+func DialConnManual(conn Conn, ocb *OCB) (*Client, error) {
+	c := &Client{
+		conn:             conn,
+		transport:        NewTransport(ocb, false),
+		ocb:              ocb,
+		outputC:          make(chan struct{}, 1),
+		done:             make(chan struct{}),
+		sentActionCounts: make(map[uint64]int),
+	}
+
+	c.wg.Add(1)
+	go c.recvLoop()
+
+	// Send initial keepalive to associate with the server.
+	c.transport.ForceNextSend()
+	c.tick()
+
+	return c, nil
+}
+
 // Send sends keystrokes to the server.
 func (c *Client) Send(keys []byte) {
 	c.actionsMu.Lock()
@@ -141,10 +170,23 @@ func (c *Client) Transport() *Transport {
 	return c.transport
 }
 
+// Tick flushes pending actions as mosh datagrams. Call this periodically
+// from an external timer (e.g., JS setInterval in WASM) when the internal
+// sendLoop is not used.
+func (c *Client) Tick() {
+	c.tick()
+}
+
 func (c *Client) tick() {
-	// Batch: compute diff from accumulated actions.
+	// Only create a new state when the server has acked the previous one.
+	// This matches the Dart mosh client's behavior: at most one unacked
+	// state in flight. Keystrokes accumulate until the ack arrives, then
+	// the next tick sends them all as a single new state.
+	// Without this check, SetPending resets diffSent, causing Tick() to
+	// increment sentNum for the same cumulative payload — the server
+	// applies duplicate keystrokes.
 	c.actionsMu.Lock()
-	if c.dirty {
+	if c.dirty && c.transport.AckedByRemote() >= c.transport.SentNum() {
 		c.dirty = false
 		c.processAcksLocked()
 		newActions := c.actions[c.ackedActionCount:]

cmd/mosh-wasm/main.go

@@ -0,0 +1,135 @@
+//go:build js && wasm
+
+package main
+
+import (
+	"encoding/base64"
+	"fmt"
+	"sync"
+	"syscall/js"
+
+	mosh "github.com/unixshells/mosh-go"
+)
+
+func main() {
+	js.Global().Set("moshConnect", js.FuncOf(moshConnect))
+	select {} // keep alive
+}
+
+// moshConnect(url, key) → Promise<MoshSession>
+// url: WebTransport URL (e.g., "https://relay.example.com/mosh/user/device")
+// key: base64-encoded mosh key
+func moshConnect(this js.Value, args []js.Value) interface{} {
+	if len(args) < 2 {
+		return reject("moshConnect requires (url, key)")
+	}
+
+	url := args[0].String()
+	key := args[1].String()
+
+	handler := js.FuncOf(func(this js.Value, pargs []js.Value) interface{} {
+		resolve := pargs[0]
+		rejectFn := pargs[1]
+
+		go func() {
+			session, err := newSession(url, key)
+			if err != nil {
+				rejectFn.Invoke(err.Error())
+				return
+			}
+			resolve.Invoke(session.jsObject())
+		}()
+
+		return nil
+	})
+
+	return js.Global().Get("Promise").New(handler)
+}
+
+type session struct {
+	client *mosh.Client
+	conn   *wtConn
+	mu     sync.Mutex
+	closed bool
+}
+
+func newSession(url, key string) (*session, error) {
+	for len(key)%4 != 0 {
+		key += "="
+	}
+	rawKey, err := base64.StdEncoding.DecodeString(key)
+	if err != nil {
+		return nil, fmt.Errorf("bad key: %w", err)
+	}
+	ocb, err := mosh.NewOCB(rawKey)
+	if err != nil {
+		return nil, err
+	}
+
+	conn, err := dialWebTransport(url)
+	if err != nil {
+		return nil, err
+	}
+
+	// Use DialConnManual — no internal sendLoop. JS drives Tick() via setInterval.
+	client, err := mosh.DialConnManual(conn, ocb)
+	if err != nil {
+		conn.Close()
+		return nil, err
+	}
+
+	return &session{client: client, conn: conn}, nil
+}
+
+func (s *session) jsObject() js.Value {
+	obj := js.Global().Get("Object").New()
+
+	obj.Set("send", js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+		if len(args) < 1 {
+			return nil
+		}
+		s.client.Send([]byte(args[0].String()))
+		s.client.Tick()
+		return nil
+	}))
+
+	obj.Set("resize", js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+		if len(args) < 2 {
+			return nil
+		}
+		s.client.Resize(uint16(args[0].Int()), uint16(args[1].Int()))
+		s.client.Tick()
+		return nil
+	}))
+
+	// tick() — flush pending keystrokes + keepalive/retransmit.
+	obj.Set("tick", js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+		s.client.Tick()
+		return nil
+	}))
+
+	// poll() — check for output. Called by JS setInterval.
+	// Returns string if output available, null otherwise.
+	// Using JS-driven polling avoids Go goroutine scheduler starvation.
+	obj.Set("poll", js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+		out := s.client.Recv(0)
+		if out == nil {
+			return js.Null()
+		}
+		return string(out)
+	}))
+
+	obj.Set("close", js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+		s.mu.Lock()
+		s.closed = true
+		s.mu.Unlock()
+		s.client.Close()
+		return nil
+	}))
+
+	return obj
+}
+
+func reject(msg string) js.Value {
+	return js.Global().Get("Promise").Call("reject", msg)
+}

cmd/mosh-wasm/webtransport.go

@@ -0,0 +1,158 @@
+//go:build js && wasm
+
+package main
+
+import (
+	"errors"
+	"sync"
+	"syscall/js"
+	"time"
+)
+
+// wtConn wraps a browser WebTransport connection as a mosh.Conn.
+type wtConn struct {
+	transport js.Value
+	writer    js.Value // persistent writable stream writer
+	incoming  chan []byte
+	done      chan struct{}
+	once      sync.Once
+
+	mu       sync.Mutex
+	deadline time.Time
+}
+
+func dialWebTransport(url string) (*wtConn, error) {
+	wt := js.Global().Get("WebTransport").New(url)
+
+	readyCh := make(chan error, 1)
+	onReady := js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+		readyCh <- nil
+		return nil
+	})
+	onFail := js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+		msg := "WebTransport connection failed"
+		if len(args) > 0 && !args[0].IsUndefined() && !args[0].IsNull() {
+			if m := args[0].Get("message"); !m.IsUndefined() {
+				msg = m.String()
+			}
+		}
+		readyCh <- errors.New(msg)
+		return nil
+	})
+	wt.Get("ready").Call("then", onReady, onFail)
+
+	select {
+	case err := <-readyCh:
+		if err != nil {
+			onReady.Release()
+			onFail.Release()
+			return nil, err
+		}
+	case <-time.After(10 * time.Second):
+		onReady.Release()
+		onFail.Release()
+		wt.Call("close")
+		return nil, errors.New("WebTransport connect timeout")
+	}
+	onReady.Release()
+	onFail.Release()
+
+	writer := wt.Get("datagrams").Get("writable").Call("getWriter")
+
+	c := &wtConn{
+		transport: wt,
+		writer:    writer,
+		incoming:  make(chan []byte, 256),
+		done:      make(chan struct{}),
+	}
+
+	// Start reading datagrams via a persistent JS callback.
+	c.startReader()
+
+	return c, nil
+}
+
+// startReader sets up a self-chaining read loop using persistent callbacks.
+func (c *wtConn) startReader() {
+	reader := c.transport.Get("datagrams").Get("readable").Call("getReader")
+
+	var onData, onErr js.Func
+	var readNext js.Func
+
+	readNext = js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+		reader.Call("read").Call("then", onData, onErr)
+		return nil
+	})
+
+	onData = js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+		result := args[0]
+		if result.Get("done").Bool() {
+			return nil
+		}
+		value := result.Get("value")
+		buf := make([]byte, value.Get("byteLength").Int())
+		js.CopyBytesToGo(buf, js.Global().Get("Uint8Array").New(value))
+		select {
+		case c.incoming <- buf:
+		default:
+		}
+		// Defer next read to next event loop tick so Go goroutines can process.
+		js.Global().Call("setTimeout", readNext, 0)
+		return nil
+	})
+
+	onErr = js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+		// Stream error or closed — stop reading.
+		return nil
+	})
+
+	// Start first read.
+	readNext.Invoke()
+}
+
+func (c *wtConn) Read(b []byte) (int, error) {
+	c.mu.Lock()
+	deadline := c.deadline
+	c.mu.Unlock()
+
+	var timer <-chan time.Time
+	if !deadline.IsZero() {
+		d := time.Until(deadline)
+		if d <= 0 {
+			return 0, errors.New("i/o timeout")
+		}
+		timer = time.After(d)
+	}
+
+	select {
+	case <-c.done:
+		return 0, errors.New("connection closed")
+	case data := <-c.incoming:
+		n := copy(b, data)
+		return n, nil
+	case <-timer:
+		return 0, errors.New("i/o timeout")
+	}
+}
+
+func (c *wtConn) Write(b []byte) (int, error) {
+	arr := js.Global().Get("Uint8Array").New(len(b))
+	js.CopyBytesToJS(arr, b)
+	c.writer.Call("write", arr)
+	return len(b), nil
+}
+
+func (c *wtConn) SetReadDeadline(t time.Time) error {
+	c.mu.Lock()
+	c.deadline = t
+	c.mu.Unlock()
+	return nil
+}
+
+func (c *wtConn) Close() error {
+	c.once.Do(func() {
+		close(c.done)
+		c.transport.Call("close")
+	})
+	return nil
+}

cmd/remote-test/main.go

@@ -0,0 +1,133 @@
+package main
+
+import (
+	"encoding/base64"
+	"fmt"
+	"net"
+	"os"
+	"strconv"
+	"time"
+	"unicode/utf8"
+
+	mosh "github.com/unixshells/mosh-go"
+)
+
+func main() {
+	if len(os.Args) != 4 {
+		fmt.Fprintf(os.Stderr, "Usage: %s <host> <port> <base64key>\n", os.Args[0])
+		os.Exit(1)
+	}
+
+	host := os.Args[1]
+	port, _ := strconv.Atoi(os.Args[2])
+	key, err := base64.StdEncoding.DecodeString(os.Args[3])
+	if err != nil {
+		// Try raw base64 without padding.
+		key, err = base64.RawStdEncoding.DecodeString(os.Args[2+1])
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "bad key: %v\n", err)
+			os.Exit(1)
+		}
+	}
+
+	ocb, err := mosh.NewOCB(key)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "ocb: %v\n", err)
+		os.Exit(1)
+	}
+
+	transport := mosh.NewTransport(ocb, false) // client mode
+
+	// Resolve host.
+	addrs, err := net.ResolveUDPAddr("udp", fmt.Sprintf("%s:%d", host, port))
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "resolve: %v\n", err)
+		os.Exit(1)
+	}
+
+	conn, err := net.DialUDP("udp", nil, addrs)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "dial: %v\n", err)
+		os.Exit(1)
+	}
+	defer conn.Close()
+
+	flush := func() {
+		datagrams := transport.Tick()
+		for _, dg := range datagrams {
+			conn.Write(dg)
+		}
+	}
+
+	// Read incoming in background.
+	go func() {
+		buf := make([]byte, 65536)
+		for {
+			n, err := conn.Read(buf)
+			if err != nil {
+				return
+			}
+			diff := transport.Recv(buf[:n])
+			if diff == nil || len(diff) == 0 {
+				continue
+			}
+
+			instrs, err := mosh.UnmarshalHostMessage(diff)
+			if err != nil {
+				fmt.Printf("unmarshal error: %v\n", err)
+				continue
+			}
+			for _, hi := range instrs {
+				if len(hi.Hoststring) > 0 {
+					text := string(hi.Hoststring)
+					if !utf8.ValidString(text) {
+						text = fmt.Sprintf("(%d raw bytes)", len(hi.Hoststring))
+					}
+					fmt.Printf("=== hoststring (%d bytes) ===\n%s\n=== end ===\n", len(hi.Hoststring), text)
+				}
+				if hi.Width > 0 || hi.Height > 0 {
+					fmt.Printf("=== resize: %dx%d ===\n", hi.Width, hi.Height)
+				}
+			}
+		}
+	}()
+
+	// Tick timer.
+	ticker := time.NewTicker(20 * time.Millisecond)
+	go func() {
+		for range ticker.C {
+			flush()
+		}
+	}()
+
+	// Initial keepalive.
+	fmt.Println("Sending initial keepalive...")
+	transport.ForceNextSend()
+	flush()
+
+	// Resize.
+	fmt.Println("Sending resize 80x24...")
+	resize := mosh.MarshalUserMessage([]mosh.UserInstruction{
+		{Width: 80, Height: 24},
+	})
+	transport.SetPending(resize)
+	flush()
+
+	// Wait for initial output.
+	fmt.Println("Waiting 5 seconds for initial output...")
+	time.Sleep(5 * time.Second)
+
+	// Send ls + Enter.
+	fmt.Println("\nSending ls + Enter...")
+	keystroke := mosh.MarshalUserMessage([]mosh.UserInstruction{
+		{Keys: []byte("ls\r")},
+	})
+	transport.SetPending(keystroke)
+	flush()
+
+	// Wait for response.
+	time.Sleep(3 * time.Second)
+
+	fmt.Println("\nDone.")
+	ticker.Stop()
+}

conn.go

@@ -0,0 +1,36 @@
+package mosh
+
+import "time"
+
+// Conn is a datagram-oriented connection used by the mosh transport.
+// UDP, WebTransport, and other datagram transports implement this interface.
+type Conn interface {
+	// Read reads a datagram. Returns the number of bytes read.
+	// Must respect deadlines set by SetReadDeadline.
+	Read(b []byte) (int, error)
+
+	// Write writes a datagram.
+	Write(b []byte) (int, error)
+
+	// SetReadDeadline sets the deadline for Read calls.
+	SetReadDeadline(t time.Time) error
+
+	// Close closes the connection.
+	Close() error
+}
+
+const (
+	// Direction bits in the 64-bit nonce header.
+	dirToServer = uint64(0)          // client → server
+	dirToClient = uint64(1) << 63    // server → client
+	seqMask     = ^(uint64(1) << 63) // lower 63 bits
+
+	// Minimum wire datagram: 8 (nonce) + 16 (tag) = 24 bytes.
+	minDatagram = 24
+
+	// Maximum payload per UDP datagram.
+	maxPayload = 16384
+
+	// Tick interval matches mosh's SEND_MINDELAY (8ms) for keystroke batching.
+	tickInterval = 8 * time.Millisecond
+)

framebuffer.go

@@ -1,3 +1,5 @@
+//go:build !js
+
 package mosh
 
 import (

server.go

@@ -1,3 +1,5 @@
+//go:build !js
+
 package mosh
 
 import (
@@ -18,20 +20,6 @@ import (
 )
 
 const (
-	// Direction bits in the 64-bit nonce header.
-	dirToServer = uint64(0)          // client → server
-	dirToClient = uint64(1) << 63    // server → client
-	seqMask     = ^(uint64(1) << 63) // lower 63 bits
-
-	// Minimum wire datagram: 8 (nonce) + 16 (tag) = 24 bytes.
-	minDatagram = 24
-
-	// Maximum payload per UDP datagram.
-	maxPayload = 16384
-
-	// Tick interval matches mosh's SEND_MINDELAY (8ms) for keystroke batching.
-	tickInterval = 8 * time.Millisecond
-
 	// Server gives up waiting for client after this.
 	associationTimeout = 60 * time.Second
 
@@ -270,6 +258,9 @@ func (s *Server) readIO(r io.Reader, out chan<- []byte) {
 }
 
 func (s *Server) mainLoopRW(rw io.Writer, resize func(cols, rows uint16), ioOutput <-chan []byte, userInput <-chan UserInstruction) {
+	// Wait for the first client datagram. If no client connects within
+	// 60 seconds, this was a failed setup — shut down to release resources.
+	deadline := time.After(associationTimeout)
 	for {
 		s.mu.Lock()
 		addr := s.clientAddr
@@ -280,6 +271,9 @@ func (s *Server) mainLoopRW(rw io.Writer, resize func(cols, rows uint16), ioOutp
 		select {
 		case <-s.done:
 			return
+		case <-deadline:
+			close(s.done)
+			return
 		case <-time.After(100 * time.Millisecond):
 		}
 	}
@@ -369,7 +363,9 @@ func (s *Server) Close() {
 // snapshots the framebuffer, diffs against the last-acked state, and
 // sends CUP-based updates via the transport.
 func (s *Server) mainLoop(ptyOutput <-chan []byte, userInput <-chan UserInstruction) {
-	// Wait for client to connect before processing.
+	// Wait for the first client datagram. If no client connects within
+	// 60 seconds, this was a failed setup — shut down to release resources.
+	deadline := time.After(associationTimeout)
 	for {
 		s.mu.Lock()
 		addr := s.clientAddr
@@ -380,6 +376,9 @@ func (s *Server) mainLoop(ptyOutput <-chan []byte, userInput <-chan UserInstruct
 		select {
 		case <-s.done:
 			return
+		case <-deadline:
+			close(s.done)
+			return
 		case <-time.After(100 * time.Millisecond):
 		}
 	}

server_test.go

@@ -1,3 +1,5 @@
+//go:build !js
+
 package mosh
 
 import (