Backport: Disable real-time collaboration when revisions are off

src/wp-admin/options-writing.php

@@ -112,15 +112,25 @@
 <tr>
 <th scope="row"><?php _e( 'Collaboration' ); ?></th>
 <td>
-	<?php if ( wp_is_collaboration_allowed() ) : ?>
+	<?php if ( ! wp_is_collaboration_allowed() ) : ?>
+		<div class="notice notice-warning inline">
+			<p><?php _e( '<strong>Note:</strong> Real-time collaboration has been disabled.' ); ?></p>
+		</div>
+	<?php elseif ( ! wp_revisions_are_globally_supported() ) : ?>
+		<div class="notice notice-warning inline">
+			<p><?php _e( '<strong>Note:</strong> Real-time collaboration requires post revisions. Enable revisions in your site configuration to use collaboration.' ); ?></p>
+		</div>
+		<p>
+			<label for="wp_collaboration_enabled">
+				<input name="wp_collaboration_enabled_disabled" type="checkbox" id="wp_collaboration_enabled" disabled="disabled" <?php checked( '1', (bool) get_option( 'wp_collaboration_enabled' ) ); ?> />
+				<?php _e( "Enable early access to real-time collaboration. Real-time collaboration may affect your website's performance." ); ?>
+			</label>
+		</p>
+	<?php else : ?>
 		<label for="wp_collaboration_enabled">
 			<input name="wp_collaboration_enabled" type="checkbox" id="wp_collaboration_enabled" value="1" <?php checked( '1', (bool) get_option( 'wp_collaboration_enabled' ) ); ?> />
 			<?php _e( "Enable early access to real-time collaboration. Real-time collaboration may affect your website's performance." ); ?>
 		</label>
-	<?php else : ?>
-		<div class="notice notice-warning inline">
-			<p><?php _e( '<strong>Note:</strong> Real-time collaboration has been disabled.' ); ?></p>
-		</div>
 	<?php endif; ?>
 </td>
 </tr>

src/wp-includes/collaboration.php

@@ -12,6 +12,8 @@
  * If the WP_ALLOW_COLLABORATION constant is false,
  * collaboration is always disabled regardless of the database option.
  * Otherwise, falls back to the 'wp_collaboration_enabled' option.
+ * Collaboration is also disabled when post revisions are turned off
+ * site-wide (see {@see wp_revisions_are_globally_supported()}).
  *
  * @since 7.0.0
  *
@@ -20,7 +22,8 @@
 function wp_is_collaboration_enabled() {
 	return (
 		wp_is_collaboration_allowed() &&
-		(bool) get_option( 'wp_collaboration_enabled' )
+		(bool) get_option( 'wp_collaboration_enabled' ) &&
+		wp_revisions_are_globally_supported()
 	);
 }
 
@@ -55,6 +58,29 @@ function wp_is_collaboration_allowed() {
 	return WP_ALLOW_COLLABORATION;
 }
 
+/**
+ * Determines whether post revisions are enabled site-wide.
+ *
+ * When {@see WP_POST_REVISIONS} is false or 0, WordPress does not store
+ * revisions. Real-time collaboration depends on revision-related flows and
+ * must remain off in that configuration.
+ *
+ * @since 7.0.0
+ *
+ * @return bool Whether revisions are globally supported.
+ */
+function wp_revisions_are_globally_supported() {
+	if ( ! defined( 'WP_POST_REVISIONS' ) ) {
+		return true;
+	}
+
+	if ( false === WP_POST_REVISIONS ) {
+		return false;
+	}
+
+	return 0 !== (int) WP_POST_REVISIONS;
+}
+
 /**
  * Injects the real-time collaboration setting into a global variable.
  *

tests/phpunit/tests/collaboration/wpRevisionsGloballySupported.php

@@ -0,0 +1,35 @@
+<?php
+/**
+ * Tests for revision-based gating of real-time collaboration.
+ *
+ * @package WordPress
+ * @subpackage Collaboration
+ *
+ * @group collaboration
+ *
+ * @covers ::wp_revisions_are_globally_supported
+ * @covers ::wp_is_collaboration_enabled
+ */
+class Tests_Collaboration_WpRevisionsGloballySupported extends WP_UnitTestCase {
+
+	/**
+	 * When WP_POST_REVISIONS is unset, revisions behave as enabled site-wide.
+	 *
+	 * @ticket 77499
+	 */
+	public function test_wp_revisions_are_globally_supported_defaults_to_true() {
+		$this->assertTrue( wp_revisions_are_globally_supported() );
+	}
+
+	/**
+	 * Collaboration follows global revision availability when the option is on.
+	 *
+	 * @ticket 77499
+	 */
+	public function test_wp_is_collaboration_enabled_true_when_option_enabled_and_revisions_supported() {
+		update_option( 'wp_collaboration_enabled', 1 );
+
+		$this->assertTrue( wp_revisions_are_globally_supported() );
+		$this->assertTrue( wp_is_collaboration_enabled() );
+	}
+}