Better websocket reconnection handling (#2960)

Right now it's fairly easy to encounter a situation when a tab coming online wouldn't recognise that the connection can now be reestablished for a while. This PR cleans up reconnection logic, reenables tests, and makes sure we get online as robustly as possible. ### Change Type - [x] `patch` — Bug fix ### Test Plan 1. Check that reconnection works as expected - [x] End to end tests --------- Co-authored-by: David Sheldrick <d.j.sheldrick@gmail.com>
2024-03-04 16:48:14 +00:00 · 2024-03-04 16:48:14 +00:00 · c3e8628680
commit c3e8628680
parent f0f133fdd2
7 changed files with 415 additions and 167 deletions
--- a/apps/dotcom/.eslintignore
+++ b/apps/dotcom/.eslintignore
@ -0,0 +1 @@
+jestResolver.js
--- a/apps/dotcom/jestResolver.js
+++ b/apps/dotcom/jestResolver.js
@ -0,0 +1,25 @@
+// HACK: `ws` has an import map mapping browser context to a dummy implementation that just
+// throws an error, because it's impossible to create a websocket server in the browser
+// and `ws` tries to be helpful. Unfortunately, it doesn't work well in our tests:
+// we run Jest in jsdom context, because we test browser APIs, but this causes Jest
+// to select the browser version of the package, which makes it impossible to run a test
+// websocket server.
+//
+// The solution is to override `ws` exports in the Jest resolver to point to the node version
+// regardless.
+//
+// An additional complication is that Jest seems to expect the resolver to be a CommonJS module,
+// so this module is CommonJS despite the rest of the codebase being ESM.
+//
+// see https://jestjs.io/docs/configuration#resolver-string for docs
+module.exports = function jestResolver(path, options) {
+	return options.defaultResolver(path, {
+		...options,
+		packageFilter: (pkg) => {
+			if (path === 'ws') {
+				pkg.exports['.']['browser'] = './index.js'
+			}
+			return pkg
+		},
+	})
+}
--- a/apps/dotcom/package.json
+++ b/apps/dotcom/package.json
@ -24,6 +24,7 @@
 		"@sentry/react": "^7.77.0",
 		"@tldraw/assets": "workspace:*",
 		"@tldraw/tlsync": "workspace:*",
+		"@tldraw/utils": "workspace:*",
 		"@vercel/analytics": "^1.1.1",
 		"browser-fs-access": "^0.33.0",
 		"idb": "^7.1.1",
@ -52,6 +53,7 @@
 		"ws": "^8.16.0"
 	},
 	"jest": {
+		"resolver": "<rootDir>/jestResolver.js",
 		"preset": "config/jest/node",
 		"roots": [
 			"<rootDir>"
--- a/apps/dotcom/src/utils/remote-sync/ClientWebSocketAdapter.test.ts
+++ b/apps/dotcom/src/utils/remote-sync/ClientWebSocketAdapter.test.ts
@ -1,6 +1,8 @@
-import { TLSYNC_PROTOCOL_VERSION } from '@tldraw/tlsync'
-import * as ws from 'ws'
-import { ClientWebSocketAdapter } from './ClientWebSocketAdapter'
+import { TLSocketClientSentEvent, TLSYNC_PROTOCOL_VERSION } from '@tldraw/tlsync'
+import { TLRecord } from 'tldraw'
+import { ClientWebSocketAdapter, INACTIVE_MIN_DELAY } from './ClientWebSocketAdapter'
+// NOTE: there is a hack in apps/dotcom/jestResolver.js to make this import work
+import { WebSocketServer, WebSocket as WsWebSocket } from 'ws'

 async function waitFor(predicate: () => boolean) {
 	let safety = 0
@ -20,19 +22,16 @@ async function waitFor(predicate: () => boolean) {

 jest.useFakeTimers()

-// TODO: unskip this test. It accidentally got disabled a long time ago when we moved this file into
-// the dotcom folder which didn't have testing set up at the time. We need to spend some time fixing
-// it before it can be re-enabled.
-describe.skip(ClientWebSocketAdapter, () => {
+describe(ClientWebSocketAdapter, () => {
 	let adapter: ClientWebSocketAdapter
-	let wsServer: ws.Server
-	let connectedWs: ws.WebSocket
-	const connectMock = jest.fn<void, [socket: ws.WebSocket]>((socket) => {
-		connectedWs = socket
+	let wsServer: WebSocketServer
+	let connectedServerSocket: WsWebSocket
+	const connectMock = jest.fn<void, [socket: WsWebSocket]>((socket) => {
+		connectedServerSocket = socket
 	})
 	beforeEach(() => {
 		adapter = new ClientWebSocketAdapter(() => 'ws://localhost:2233')
-		wsServer = new ws.Server({ port: 2233 })
+		wsServer = new WebSocketServer({ port: 2233 })
 		wsServer.on('connection', connectMock)
 	})
 	afterEach(() => {
@ -59,12 +58,15 @@ describe.skip(ClientWebSocketAdapter, () => {
 		adapter._ws?.onerror?.({} as any)
 		expect(adapter.connectionStatus).toBe('error')
 	})
-	it('should try to reopen the connection if there was an error', () => {
-		const prevWes = adapter._ws
-		adapter._ws?.onerror?.({} as any)
-		jest.advanceTimersByTime(1000)
-		expect(adapter._ws).not.toBe(prevWes)
-		expect(adapter._ws?.readyState).toBe(WebSocket.CONNECTING)
+	it('should try to reopen the connection if there was an error', async () => {
+		await waitFor(() => adapter._ws?.readyState === WebSocket.OPEN)
+		expect(adapter._ws).toBeTruthy()
+		const prevClientSocket = adapter._ws
+		const prevServerSocket = connectedServerSocket
+		prevServerSocket.terminate()
+		await waitFor(() => connectedServerSocket !== prevServerSocket)
+		expect(adapter._ws).not.toBe(prevClientSocket)
+		expect(adapter._ws?.readyState).toBe(WebSocket.OPEN)
 	})
 	it('should transition to online if a retry succeeds', async () => {
 		adapter._ws?.onerror?.({} as any)
@ -72,6 +74,7 @@ describe.skip(ClientWebSocketAdapter, () => {
 		expect(adapter.connectionStatus).toBe('online')
 	})
 	it('should call .close on the underlying socket if .close is called before the socket opens', async () => {
+		await waitFor(() => adapter._ws?.readyState === WebSocket.OPEN)
 		const closeSpy = jest.spyOn(adapter._ws!, 'close')
 		adapter.close()
 		await waitFor(() => closeSpy.mock.calls.length > 0)
@ -79,39 +82,37 @@ describe.skip(ClientWebSocketAdapter, () => {
 	})
 	it('should transition to offline if the server disconnects', async () => {
 		await waitFor(() => adapter._ws?.readyState === WebSocket.OPEN)
-		connectedWs.terminate()
+		connectedServerSocket.terminate()
 		await waitFor(() => adapter._ws?.readyState === WebSocket.CLOSED)
 		expect(adapter.connectionStatus).toBe('offline')
 	})
 	it('retries to connect if the server disconnects', async () => {
 		await waitFor(() => adapter._ws?.readyState === WebSocket.OPEN)
-		connectedWs.terminate()
+		connectedServerSocket.terminate()
 		await waitFor(() => adapter._ws?.readyState === WebSocket.CLOSED)
 		expect(adapter.connectionStatus).toBe('offline')
 		await waitFor(() => adapter._ws?.readyState === WebSocket.OPEN)
 		expect(adapter.connectionStatus).toBe('online')
-		connectedWs.terminate()
+		connectedServerSocket.terminate()
 		await waitFor(() => adapter._ws?.readyState === WebSocket.CLOSED)
 		expect(adapter.connectionStatus).toBe('offline')
 		await waitFor(() => adapter._ws?.readyState === WebSocket.OPEN)
 		expect(adapter.connectionStatus).toBe('online')
 	})

-	it('closes the socket if the window goes offline and attempts to reconnect', async () => {
-		await waitFor(() => adapter._ws?.readyState === WebSocket.OPEN)
-		const closeSpy = jest.spyOn(adapter._ws!, 'close')
-		window.dispatchEvent(new Event('offline'))
-		expect(closeSpy).toHaveBeenCalled()
-		await waitFor(() => adapter._ws?.readyState === WebSocket.OPEN)
-	})
-
-	it('attempts to reconnect early if the window comes back online', async () => {
+	it('attempts to reconnect early if the tab becomes active', async () => {
 		await waitFor(() => adapter._ws?.readyState === WebSocket.OPEN)
+		const hiddenMock = jest.spyOn(document, 'hidden', 'get')
+		hiddenMock.mockReturnValue(true)
+		// it's necessary to close the socket, as otherwise the websocket might stay half-open
+		connectedServerSocket.close()
 		wsServer.close()
-		window.dispatchEvent(new Event('offline'))
-		adapter._reconnectTimeout.intervalLength = 50000
-		window.dispatchEvent(new Event('online'))
-		expect(adapter._reconnectTimeout.intervalLength).toBeLessThan(1000)
+		await waitFor(() => adapter._ws?.readyState !== WebSocket.OPEN)
+		expect(adapter._reconnectManager.intendedDelay).toBeGreaterThanOrEqual(INACTIVE_MIN_DELAY)
+		hiddenMock.mockReturnValue(false)
+		document.dispatchEvent(new Event('visibilitychange'))
+		expect(adapter._reconnectManager.intendedDelay).toBeLessThan(INACTIVE_MIN_DELAY)
+		hiddenMock.mockRestore()
 	})

 	it('supports receiving messages', async () => {
@ -125,8 +126,7 @@ describe.skip(ClientWebSocketAdapter, () => {
 		expect(onMessage).toHaveBeenCalledWith({ type: 'message', data: 'hello' })
 	})

-	// TODO: this is failing on github actions, investigate
-	it.skip('supports sending messages', async () => {
+	it('supports sending messages', async () => {
 		const onMessage = jest.fn()
 		connectMock.mockImplementationOnce((ws) => {
 			ws.on('message', onMessage)
@ -134,19 +134,19 @@ describe.skip(ClientWebSocketAdapter, () => {

 		await waitFor(() => adapter._ws?.readyState === WebSocket.OPEN)

-		adapter.sendMessage({
+		const message: TLSocketClientSentEvent<TLRecord> = {
 			type: 'connect',
 			connectRequestId: 'test',
 			schema: { schemaVersion: 0, storeVersion: 0, recordVersions: {} },
 			protocolVersion: TLSYNC_PROTOCOL_VERSION,
 			lastServerClock: 0,
-		})
+		}
+
+		adapter.sendMessage(message)

 		await waitFor(() => onMessage.mock.calls.length === 1)

-		expect(onMessage.mock.calls[0][0].toString()).toBe(
-			'{"type":"connect","instanceId":"test","lastServerClock":0}'
-		)
+		expect(JSON.parse(onMessage.mock.calls[0][0].toString())).toEqual(message)
 	})

 	it('signals status changes', async () => {
@ -154,12 +154,12 @@ describe.skip(ClientWebSocketAdapter, () => {
 		adapter.onStatusChange(onStatusChange)
 		await waitFor(() => adapter._ws?.readyState === WebSocket.OPEN)
 		expect(onStatusChange).toHaveBeenCalledWith('online')
-		connectedWs.terminate()
+		connectedServerSocket.terminate()
 		await waitFor(() => adapter._ws?.readyState === WebSocket.CLOSED)
 		expect(onStatusChange).toHaveBeenCalledWith('offline')
 		await waitFor(() => adapter._ws?.readyState === WebSocket.OPEN)
 		expect(onStatusChange).toHaveBeenCalledWith('online')
-		connectedWs.terminate()
+		connectedServerSocket.terminate()
 		await waitFor(() => adapter._ws?.readyState === WebSocket.CLOSED)
 		expect(onStatusChange).toHaveBeenCalledWith('offline')
 		await waitFor(() => adapter._ws?.readyState === WebSocket.OPEN)
--- a/apps/dotcom/src/utils/remote-sync/ClientWebSocketAdapter.ts
+++ b/apps/dotcom/src/utils/remote-sync/ClientWebSocketAdapter.ts
@ -6,137 +6,160 @@ import {
 	TLSocketClientSentEvent,
 	TLSocketServerSentEvent,
 } from '@tldraw/tlsync'
+import { assert } from '@tldraw/utils'
 import { atom, Atom, TLRecord } from 'tldraw'

-function windowListen(...args: Parameters<typeof window.addEventListener>) {
-	window.addEventListener(...args)
+function listenTo<T extends EventTarget>(target: T, event: string, handler: () => void) {
+	target.addEventListener(event, handler)
 	return () => {
-		window.removeEventListener(...args)
+		target.removeEventListener(event, handler)
 	}
 }

 function debug(...args: any[]) {
 	// @ts-ignore
 	if (typeof window !== 'undefined' && window.__tldraw_socket_debug) {
+		const now = new Date()
 		// eslint-disable-next-line no-console
-		console.log(...args, new Error().stack)
+		console.log(
+			`${now.getHours()}:${now.getMinutes()}:${now.getSeconds()}.${now.getMilliseconds()}`,
+			...args
+			//, new Error().stack
+		)
 	}
 }

+// NOTE: ClientWebSocketAdapter requires its users to implement their own connection loss
+//       detection, for example by regularly pinging the server and .restart()ing
+//       the connection when a number of pings goes unanswered. Without this mechanism,
+//       we might not be able to detect the websocket connection going down in a timely manner
+//       (it will probably time out on outgoing data packets at some point).
+//
+//       This is by design. Whilst the Websocket protocol specifies protocol-level pings,
+//       they don't seem to be surfaced in browser APIs and can't be relied on. Therefore,
+//       pings need to be implemented one level up, on the application API side, which for our
+//       codebase means whatever code that uses ClientWebSocketAdapter.
+
 export class ClientWebSocketAdapter implements TLPersistentClientSocket<TLRecord> {
 	_ws: WebSocket | null = null

-	wasManuallyClosed = false
+	isDisposed = false

-	disposables: (() => void)[] = []
+	readonly _reconnectManager: ReconnectManager

+	// TODO: .close should be a project-wide interface with a common contract (.close()d thing
+	//       can only be garbage collected, and can't be used anymore)
 	close() {
-		this.wasManuallyClosed = true
-		this.disposables.forEach((d) => d())
-		this._reconnectTimeout.clear()
-		if (this._ws?.readyState === WebSocket.OPEN) {
-			debug('close d')
-			this._ws.close()
-		}
-	}
-
-	constructor(private getUri: () => Promise<string> | string) {
-		this.disposables.push(
-			windowListen('online', () => {
-				debug('window online')
-				if (this.connectionStatus !== 'online') {
-					this._reconnectTimeout.clear()
-					this._attemptReconnect()
-				}
-			}),
-			windowListen('offline', () => {
-				debug('window offline')
-				if (this.connectionStatus === 'online') {
+		this.isDisposed = true
+		this._reconnectManager.close()
+		//  WebSocket.close() is idempotent
 		this._ws?.close()
-					this._ws?.onclose?.(null as any)
 	}
-			}),
-			windowListen('pointermove', () => {
-				// if the pointer moves while we are offline, we should try to reconnect more
-				// often than every 5 mins!
-				if (this.connectionStatus !== 'online') {
-					this._reconnectTimeout.userInteractionOccurred()
+
+	constructor(getUri: () => Promise<string> | string) {
+		this._reconnectManager = new ReconnectManager(this, getUri)
 	}
-			}),
-			windowListen('keydown', () => {
-				// if the user pressed a key while we are offline, we should try to reconnect more
-				// often than every 5 mins!
-				if (this.connectionStatus !== 'online') {
-					this._reconnectTimeout.userInteractionOccurred()
+
+	private _handleConnect() {
+		debug('handleConnect')
+
+		this._connectionStatus.set('online')
+		this.statusListeners.forEach((cb) => cb('online'))
+
+		this._reconnectManager.connected()
 	}
+
+	private _handleDisconnect(reason: 'closed' | 'error' | 'manual') {
+		debug('handleDisconnect', {
+			currentStatus: this.connectionStatus,
+			reason,
 		})
-		)
-		this._reconnectTimeout.run()
+
+		let newStatus: 'offline' | 'error'
+		switch (reason) {
+			case 'closed':
+				newStatus = 'offline'
+				break
+			case 'error':
+				newStatus = 'error'
+				break
+			case 'manual':
+				newStatus = 'offline'
+				break
 		}

-	private handleDisconnect(status: Exclude<TLPersistentClientSocketStatus, 'online'>) {
-		debug('handleDisconnect', status, this.connectionStatus)
 		if (
-			// if the status is the same as before, don't do anything
-			this.connectionStatus === status ||
-			// if we receive an error we only care about it while we're in the initial state
-			(status === 'error' && this.connectionStatus === 'offline')
+			// it the status changed
+			this.connectionStatus !== newStatus &&
+			// ignore errors if we're already in the offline state
+			!(newStatus === 'error' && this.connectionStatus === 'offline')
 		) {
-			this._attemptReconnect()
-			return
-		}
-		this._connectionStatus.set(status)
-		this.statusListeners.forEach((cb) => cb(status))
-		this._reconnectTimeout.clear()
-		this._attemptReconnect()
+			this._connectionStatus.set(newStatus)
+			this.statusListeners.forEach((cb) => cb(newStatus))
 		}

-	private configureSocket() {
-		const ws = this._ws
-		if (!ws) return
+		this._reconnectManager.disconnected()
+	}
+
+	_setNewSocket(ws: WebSocket) {
+		assert(!this.isDisposed, 'Tried to set a new websocket on a disposed socket')
+		assert(
+			this._ws === null ||
+				this._ws.readyState === WebSocket.CLOSED ||
+				this._ws.readyState === WebSocket.CLOSING,
+			`Tried to set a new websocket in when the existing one was ${this._ws?.readyState}`
+		)
+		// NOTE: Sockets can stay for quite a while in the CLOSING state. This is because the transition
+		//       between CLOSING and CLOSED happens either after the closing handshake, or after a
+		//       timeout, but in either case those sockets don't need any special handling, the browser
+		//       will close them eventually. We just "orphan" such sockets and ignore their onclose/onerror.
 		ws.onopen = () => {
 			debug('ws.onopen')
-			// ws might be opened multiple times so need to check that it wasn't already supplanted
-			if (this._ws !== ws || this.wasManuallyClosed) {
-				if (ws.readyState === WebSocket.OPEN) {
-					debug('close a')
-					ws.close()
-				}
-				return
-			}
-			this._connectionStatus.set('online')
-			this.statusListeners.forEach((cb) => cb(this.connectionStatus))
-			this._reconnectTimeout.clear()
+			assert(
+				this._ws === ws,
+				"sockets must only be orphaned when they are CLOSING or CLOSED, so they can't open"
+			)
+			this._handleConnect()
 		}
 		ws.onclose = () => {
 			debug('ws.onclose')
-			this.handleDisconnect('offline')
+			if (this._ws === ws) {
+				this._handleDisconnect('closed')
+			} else {
+				debug('ignoring onclose for an orphaned socket')
+			}
 		}
 		ws.onerror = () => {
 			debug('ws.onerror')
-			this.handleDisconnect('error')
+			if (this._ws === ws) {
+				this._handleDisconnect('error')
+			} else {
+				debug('ignoring onerror for an orphaned socket')
+			}
 		}
 		ws.onmessage = (ev) => {
+			assert(
+				this._ws === ws,
+				"sockets must only be orphaned when they are CLOSING or CLOSED, so they can't receive messages"
+			)
 			const parsed = JSON.parse(ev.data.toString())
 			this.messageListeners.forEach((cb) => cb(parsed))
 		}
+
+		this._ws = ws
 	}

-	readonly _reconnectTimeout = new ExponentialBackoffTimeout(async () => {
-		debug('close b')
-		this._ws?.close()
-		this._ws = new WebSocket(await this.getUri())
-		this.configureSocket()
-	})
+	_closeSocket() {
+		if (this._ws === null) return

-	_attemptReconnect() {
-		debug('_attemptReconnect', this.wasManuallyClosed)
-		if (this.wasManuallyClosed) {
-			return
-		}
-		this._reconnectTimeout.run()
+		this._ws.close()
+		// explicitly orphan the socket to ignore its onclose/onerror, because onclose can be delayed
+		this._ws = null
+		this._handleDisconnect('manual')
 	}

+	// TLPersistentClientSocket stuff
+
 	_connectionStatus: Atom<TLPersistentClientSocketStatus | 'initial'> = atom(
 		'websocket connection status',
 		'initial'
@ -149,6 +172,8 @@ export class ClientWebSocketAdapter implements TLPersistentClientSocket<TLRecord
 	}

 	sendMessage(msg: TLSocketClientSentEvent<TLRecord>) {
+		assert(!this.isDisposed, 'Tried to send message on a disposed socket')
+
 		if (!this._ws) return
 		if (this.connectionStatus === 'online') {
 			const chunks = chunk(serializeMessage(msg))
@ -162,6 +187,8 @@ export class ClientWebSocketAdapter implements TLPersistentClientSocket<TLRecord

 	private messageListeners = new Set<(msg: TLSocketServerSentEvent<TLRecord>) => void>()
 	onReceiveMessage(cb: (val: TLSocketServerSentEvent<TLRecord>) => void) {
+		assert(!this.isDisposed, 'Tried to add message listener on a disposed socket')
+
 		this.messageListeners.add(cb)
 		return () => {
 			this.messageListeners.delete(cb)
@ -170,6 +197,8 @@ export class ClientWebSocketAdapter implements TLPersistentClientSocket<TLRecord

 	private statusListeners = new Set<(status: TLPersistentClientSocketStatus) => void>()
 	onStatusChange(cb: (val: TLPersistentClientSocketStatus) => void) {
+		assert(!this.isDisposed, 'Tried to add status listener on a disposed socket')
+
 		this.statusListeners.add(cb)
 		return () => {
 			this.statusListeners.delete(cb)
@ -177,59 +206,246 @@ export class ClientWebSocketAdapter implements TLPersistentClientSocket<TLRecord
 	}

 	restart() {
-		debug('close c')
-		this.close()
-		this.wasManuallyClosed = false
-		this._reconnectTimeout.clear()
-		this._reconnectTimeout.runNow()
+		assert(!this.isDisposed, 'Tried to restart a disposed socket')
+		debug('restarting')
+
+		this._closeSocket()
+		this._reconnectManager.maybeReconnected()
 	}
 }

-class ExponentialBackoffTimeout {
-	private timeout: NodeJS.Timeout | null = null
-	private nextScheduledRunTimestamp = 0
-	intervalLength: number
+// Those constants are exported primarily for tests
+// ACTIVE_ means the tab is active, document.hidden is false
+export const ACTIVE_MIN_DELAY = 500
+export const ACTIVE_MAX_DELAY = 2000
+// Correspondingly, here document.hidden is true. It's intended to reduce the load and battery drain
+// on client devices somewhat when they aren't looking at the tab. We don't disconnect completely
+// to minimise issues with reconnection/sync when the tab becomes visible again
+export const INACTIVE_MIN_DELAY = 1000
+export const INACTIVE_MAX_DELAY = 1000 * 60 * 5
+export const DELAY_EXPONENT = 1.5
+// this is a tradeoff between quickly detecting connections stuck in the CONNECTING state and
+// not needlessly reconnecting if the connection is just slow to establish
+export const ATTEMPT_TIMEOUT = 1000
+
+class ReconnectManager {
+	private isDisposed = false
+	private disposables: (() => void)[] = [
+		() => {
+			if (this.reconnectTimeout) clearTimeout(this.reconnectTimeout)
+			if (this.recheckConnectingTimeout) clearTimeout(this.recheckConnectingTimeout)
+		},
+	]
+	private reconnectTimeout: ReturnType<typeof setTimeout> | null = null
+	private recheckConnectingTimeout: ReturnType<typeof setTimeout> | null = null
+
+	private lastAttemptStart: number | null = null
+	intendedDelay: number = ACTIVE_MIN_DELAY
+	private state: 'pendingAttempt' | 'pendingAttemptResult' | 'delay' | 'connected'

 	constructor(
-		private cb: () => Promise<void>,
-		// five mins
-		private readonly maxIdleIntervalLength: number = 1000 * 60 * 5,
-		// five seconds
-		private readonly maxInteractiveIntervalLength: number = 1000,
-		private startIntervalLength: number = 500
+		private socketAdapter: ClientWebSocketAdapter,
+		private getUri: () => Promise<string> | string
 	) {
-		this.intervalLength = startIntervalLength
+		this.subscribeToReconnectHints()
+
+		this.disposables.push(
+			listenTo(window, 'offline', () => {
+				debug('window went offline')
+				// On the one hand, 'offline' event is not really reliable; on the other, the only
+				// alternative is to wait for pings not being delivered, which takes more than 20 seconds,
+				// which means we won't see the ClientWebSocketAdapter status change for more than
+				// 20 seconds after the tab goes offline. Our application layer must be resistent to
+				// connection restart anyway, so we can just try to reconnect and see if
+				// we're truly offline.
+				this.socketAdapter._closeSocket()
+			})
+		)
+
+		this.state = 'pendingAttempt'
+		this.intendedDelay = ACTIVE_MIN_DELAY
+		this.scheduleAttempt()
 	}

-	runNow() {
-		this.cb()
+	private subscribeToReconnectHints() {
+		this.disposables.push(
+			listenTo(window, 'online', () => {
+				debug('window went online')
+				this.maybeReconnected()
+			}),
+			listenTo(document, 'visibilitychange', () => {
+				if (!document.hidden) {
+					debug('document became visible')
+					this.maybeReconnected()
 				}
+			})
+		)

-	run() {
-		if (this.timeout) return
-		this.timeout = setTimeout(() => {
-			this.cb()
-			this.intervalLength = Math.min(this.intervalLength * 2, this.maxIdleIntervalLength)
-			if (this.timeout) {
-				clearTimeout(this.timeout)
-				this.timeout = null
-			}
-		}, this.intervalLength)
-		this.nextScheduledRunTimestamp = Date.now() + this.intervalLength
-	}
-
-	clear() {
-		this.intervalLength = this.startIntervalLength
-		if (this.timeout) {
-			clearTimeout(this.timeout)
-			this.timeout = null
+		if (Object.prototype.hasOwnProperty.call(navigator, 'connection')) {
+			const connection = (navigator as any)['connection'] as EventTarget
+			this.disposables.push(
+				listenTo(connection, 'change', () => {
+					debug('navigator.connection change')
+					this.maybeReconnected()
+				})
+			)
 		}
 	}

-	userInteractionOccurred() {
-		if (Date.now() + this.maxInteractiveIntervalLength < this.nextScheduledRunTimestamp) {
-			this.clear()
-			this.run()
+	private scheduleAttempt() {
+		assert(this.state === 'pendingAttempt')
+		debug('scheduling a connection attempt')
+		Promise.resolve(this.getUri()).then((uri) => {
+			// this can happen if the promise gets resolved too late
+			if (this.state !== 'pendingAttempt' || this.isDisposed) return
+			assert(
+				this.socketAdapter._ws?.readyState !== WebSocket.OPEN,
+				'There should be no connection attempts while already connected'
+			)
+
+			this.lastAttemptStart = Date.now()
+			this.socketAdapter._setNewSocket(new WebSocket(uri))
+			this.state = 'pendingAttemptResult'
+		})
+	}
+
+	private getMaxDelay() {
+		return document.hidden ? INACTIVE_MAX_DELAY : ACTIVE_MAX_DELAY
+	}
+
+	private getMinDelay() {
+		return document.hidden ? INACTIVE_MIN_DELAY : ACTIVE_MIN_DELAY
+	}
+
+	private clearReconnectTimeout() {
+		if (this.reconnectTimeout) {
+			clearTimeout(this.reconnectTimeout)
+			this.reconnectTimeout = null
+		}
+	}
+
+	private clearRecheckConnectingTimeout() {
+		if (this.recheckConnectingTimeout) {
+			clearTimeout(this.recheckConnectingTimeout)
+			this.recheckConnectingTimeout = null
+		}
+	}
+
+	maybeReconnected() {
+		debug('ReconnectManager.maybeReconnected')
+		// It doesn't make sense to have another check scheduled if we're already checking it now.
+		// If we have a CONNECTING check scheduled and relevant, it'll be recreated below anyway
+		this.clearRecheckConnectingTimeout()
+
+		// readyState can be CONNECTING, OPEN, CLOSING, CLOSED, or null (if getUri() is still pending)
+		if (this.socketAdapter._ws?.readyState === WebSocket.OPEN) {
+			debug('ReconnectManager.maybeReconnected: already connected')
+			// nothing to do, we're already OK
+			return
+		}
+
+		if (this.socketAdapter._ws?.readyState === WebSocket.CONNECTING) {
+			debug('ReconnectManager.maybeReconnected: connecting')
+			// We might be waiting for a TCP connection that sent SYN out and will never get it back,
+			// while a new connection appeared. On the other hand, we might have just started connecting
+			// and will succeed in a bit. Thus, we're checking how old the attempt is and retry anew
+			// if it's old enough. This by itself can delay the connection a bit, but shouldn't prevent
+			// new connections as long as `maybeReconnected` is not looped itself
+			assert(
+				this.lastAttemptStart,
+				'ReadyState=CONNECTING without lastAttemptStart should be impossible'
+			)
+			const sinceLastStart = Date.now() - this.lastAttemptStart
+			if (sinceLastStart < ATTEMPT_TIMEOUT) {
+				debug('ReconnectManager.maybeReconnected: connecting, rechecking later')
+				this.recheckConnectingTimeout = setTimeout(
+					() => this.maybeReconnected(),
+					ATTEMPT_TIMEOUT - sinceLastStart
+				)
+			} else {
+				debug('ReconnectManager.maybeReconnected: connecting, but for too long, retry now')
+				// Last connection attempt was started a while ago, it's possible that network conditions
+				// changed, and it's worth retrying to connect. `disconnected` will handle reconnection
+				//
+				// NOTE: The danger here is looping in connection attemps if connections are slow.
+				//       Make sure that `maybeReconnected` is not called in the `disconnected` codepath!
+				this.clearRecheckConnectingTimeout()
+				this.socketAdapter._closeSocket()
+			}
+
+			return
+		}
+
+		debug('ReconnectManager.maybeReconnected: closing/closed/null, retry now')
+		// readyState is CLOSING or CLOSED, or the websocket is null
+		// Restart the backoff and retry ASAP (honouring the min delay)
+		// this.state doesn't really matter, because disconnected() will handle any state correctly
+		this.intendedDelay = ACTIVE_MIN_DELAY
+		this.disconnected()
+	}
+
+	disconnected() {
+		debug('ReconnectManager.disconnected')
+		// This either means we're freshly disconnected, or the last connection attempt failed;
+		// either way, time to try again.
+
+		// Guard against delayed notifications and recheck synchronously
+		if (
+			this.socketAdapter._ws?.readyState !== WebSocket.OPEN &&
+			this.socketAdapter._ws?.readyState !== WebSocket.CONNECTING
+		) {
+			debug('ReconnectManager.disconnected: websocket is not OPEN or CONNECTING')
+			this.clearReconnectTimeout()
+
+			let delayLeft
+			if (this.state === 'connected') {
+				// it's the first sign that we got disconnected; the state will be updated below,
+				// just set the appropriate delay for now
+				this.intendedDelay = this.getMinDelay()
+				delayLeft = this.intendedDelay
+			} else {
+				delayLeft =
+					this.lastAttemptStart !== null
+						? this.lastAttemptStart + this.intendedDelay - Date.now()
+						: 0
+			}
+
+			if (delayLeft > 0) {
+				debug('ReconnectManager.disconnected: delaying, delayLeft', delayLeft)
+				// try again later
+				this.state = 'delay'
+
+				this.reconnectTimeout = setTimeout(() => this.disconnected(), delayLeft)
+			} else {
+				// not connected and not delayed, time to retry
+				this.state = 'pendingAttempt'
+
+				this.intendedDelay = Math.min(
+					this.getMaxDelay(),
+					Math.max(this.getMinDelay(), this.intendedDelay) * DELAY_EXPONENT
+				)
+				debug(
+					'ReconnectManager.disconnected: attempting a connection, next delay',
+					this.intendedDelay
+				)
+				this.scheduleAttempt()
 			}
 		}
 	}
+
+	connected() {
+		debug('ReconnectManager.connected')
+		// this notification could've been delayed, recheck synchronously
+		if (this.socketAdapter._ws?.readyState === WebSocket.OPEN) {
+			debug('ReconnectManager.connected: websocket is OPEN')
+			this.state = 'connected'
+			this.clearReconnectTimeout()
+			this.intendedDelay = ACTIVE_MIN_DELAY
+		}
+	}
+
+	close() {
+		this.disposables.forEach((d) => d())
+		this.isDisposed = true
+	}
+}
--- a/apps/dotcom/tsconfig.json
+++ b/apps/dotcom/tsconfig.json
@ -34,6 +34,9 @@
 		{
 			"path": "../../packages/tlsync"
 		},
+		{
+			"path": "../../packages/utils"
+		},
 		{
 			"path": "../../packages/validate"
 		}
--- a/yarn.lock
+++ b/yarn.lock
@ -11623,6 +11623,7 @@ __metadata:
    "@sentry/react": "npm:^7.77.0"
    "@tldraw/assets": "workspace:*"
    "@tldraw/tlsync": "workspace:*"
+    "@tldraw/utils": "workspace:*"
    "@tldraw/validate": "workspace:*"
    "@types/qrcode": "npm:^1.5.0"
    "@types/react": "npm:^18.2.47"