@@ -206,6 +206,25 @@ add_action( 'wp_presence_collaboration_ended', function( $room, $entries ) {
 }, 10, 2 );
 ```
 
+### JS Actions
+Fired through `wp.hooks`, not PHP. `presence-ping.js` is the only thing that computes these, so a consumer has to listen rather than poll for them.
+
+#### `presence-api.watchingRoom`
+Fires once, synchronously, before Heartbeat's first tick. Only on a post-edit screen for a post type with `presence` support — lets a listener tell "presence-api isn't here" apart from "here, no tick yet."
+```js
+wp.hooks.addAction( 'presence-api.watchingRoom', 'my-plugin', ( room ) => {
+    // room is 'postType/{type}:{id}'
+} );
+```
+
+#### `presence-api.collaboratorsChanged`
+Fires on the same 1-to-2+ and 2+-to-1 edges as `wp_presence_collaboration_started`/`_ended`, not on every tick.
+```js
+wp.hooks.addAction( 'presence-api.collaboratorsChanged', 'my-plugin', ( room, count ) => {
+    // count > 1 means someone besides you is in the room.
+} );
+```
+
 </details>
 
 ## REST API
@@ -265,6 +284,8 @@ No room mapping sits between the two: both use `postType/{type}:{id}`, the gramm
 
 That leaves the split: awareness and cursors inside the editor go through sync-storage; room membership everywhere else in wp-admin — plus the post-lock bridge below — stays this plugin's.
 
+The JS actions above exist for that same relationship. A consumer like Gutenberg's sync poll loop ([presence-api#444](https://github.com/WordPress/presence-api/issues/444)) can wait for `presence-api.collaboratorsChanged` instead of polling to find out whether anyone else is in the room.
+
 ## Post-lock bridge
 
 Creates presence entries alongside `_edit_lock` postmeta when a post lock is refreshed via Heartbeat. Both systems coexist.
 
@@ -1,12 +1,17 @@
 ( function ( $ ) {
-	if ( typeof wp === 'undefined' || typeof wp.heartbeat === 'undefined' ) {
+	if (
+		typeof wp === 'undefined' ||
+		typeof wp.heartbeat === 'undefined' ||
+		typeof wp.hooks === 'undefined'
+	) {
 		return;
 	}
 
 	const config = window.wpPresenceConfig || {};
 	const entries = Array.isArray( config.entries ) ? config.entries : [];
 	const frontContext = config.frontContext || null;
 	const editorPostId = parseInt( config.editorPostId, 10 ) || 0;
+	const editorRoom = config.editorRoom || '';
 	const restUrl = config.restUrl || '';
 	const nonce = config.nonce || '';
 	const idleTicks = parseInt( config.idleTicks, 10 ) || 0;
@@ -15,12 +20,22 @@
 	const backoffEnabled = idleTicks > 0 && idleInterval > 0;
 	const TTL_SAFETY_MARGIN = 15;
 
+	// Fired synchronously, ahead of Heartbeat's first tick, so a listener can
+	// tell "presence-api isn't here" apart from "here, no tick yet."
+	if ( editorRoom ) {
+		wp.hooks.doAction( 'presence-api.watchingRoom', editorRoom );
+	}
+
 	// Guards against duplicate leave() invocations.
 	let hasLeft = false;
 
 	let unchangedTicks = 0;
 	let lastOnlineHash = '';
 	let normalInterval = null;
+	// Matches wp_presence_check_collaboration_threshold()'s own default: with
+	// nothing observed yet, assume solo, so a fresh room's first tick at count
+	// 1 isn't mistaken for a 2+-to-1 edge.
+	let hasCollaborators = false;
 
 	// Reads the interval lazily (not at page load) so it reflects whatever
 	// another script, e.g. post.js's lock-refresh interval, already set.
@@ -114,6 +129,33 @@
 			}
 		} );
 
+		if ( editorRoom ) {
+			$( document ).on( 'heartbeat-tick', function ( event, data ) {
+				if (
+					! Object.prototype.hasOwnProperty.call(
+						data,
+						'presence-heartbeat-collaborators'
+					)
+				) {
+					return;
+				}
+
+				const count = data[ 'presence-heartbeat-collaborators' ];
+				const nowHasCollaborators = count > 1;
+
+				if ( nowHasCollaborators === hasCollaborators ) {
+					return;
+				}
+
+				hasCollaborators = nowHasCollaborators;
+				wp.hooks.doAction(
+					'presence-api.collaboratorsChanged',
+					editorRoom,
+					count
+				);
+			} );
+		}
+
 		if ( backoffEnabled ) {
 			// Reuses the Who's Online widget's hash exchange on every screen,
 			// not just the Dashboard, to learn whether the room changed.
 
@@ -126,6 +126,7 @@ function wp_presence_enqueue_heartbeat_ping() {
 
 	// On the post-edit screen, also occupy the per-post room.
 	$editor_post_id = 0;
+	$editor_room    = '';
 	if ( is_admin() && function_exists( 'get_current_screen' ) ) {
 		$screen = get_current_screen();
 		if ( $screen && 'post' === $screen->base ) {
@@ -134,6 +135,7 @@ function wp_presence_enqueue_heartbeat_ping() {
 				$room = wp_presence_post_room( $post->ID );
 				if ( $room ) {
 					$editor_post_id = $post->ID;
+					$editor_room    = $room;
 					$entries[]      = array(
 						'room'      => $room,
 						'client_id' => 'editor-' . $user_id,
@@ -160,24 +162,24 @@ function wp_presence_enqueue_heartbeat_ping() {
 	}
 	wp_set_presence( wp_presence_admin_room(), 'user-' . $user_id, $admin_state, $user_id );
 
-	if ( $editor_post_id ) {
-		$editor_room = wp_presence_post_room( $editor_post_id );
-		if ( $editor_room ) {
-			// No tick has carried a lock refresh yet. connectNow() on load makes
-			// that a single request, not a visible state.
-			wp_set_presence(
-				$editor_room,
-				'editor-' . $user_id,
-				wp_presence_editor_state( $screen_id, false ),
-				$user_id
-			);
-		}
+	if ( $editor_room ) {
+		// No tick has carried a lock refresh yet. connectNow() on load makes
+		// that a single request, not a visible state.
+		wp_set_presence(
+			$editor_room,
+			'editor-' . $user_id,
+			wp_presence_editor_state( $screen_id, false ),
+			$user_id
+		);
 	}
 
 	$config = array(
 		'entries'      => $entries,
 		'frontContext' => $front_context,
 		'editorPostId' => $editor_post_id,
+		// Lets presence-ping.js fire `presence-api.watchingRoom` without
+		// duplicating the postType/{type}:{id} grammar client-side.
+		'editorRoom'   => $editor_room,
 		'restUrl'      => esc_url_raw( rest_url( 'wp-presence/v1/presence' ) ),
 		'nonce'        => wp_create_nonce( 'wp_rest' ),
 		'idleTicks'    => wp_presence_get_heartbeat_idle_ticks(),
@@ -196,7 +198,7 @@ function wp_presence_enqueue_heartbeat_ping() {
 	wp_enqueue_script(
 		'wp-presence-ping',
 		WP_PRESENCE_PLUGIN_URL . 'assets/js/presence-ping.js',
-		array( 'jquery', 'heartbeat', 'wp-presence-tab-coordinator' ),
+		array( 'jquery', 'heartbeat', 'wp-hooks', 'wp-presence-tab-coordinator' ),
 		WP_PRESENCE_VERSION,
 		true
 	);