Add post_title_child_separator filter

Introduces the `post_title_child_separator` filter in
`WP_Posts_List_Table::column_title()`, allowing the hardcoded
em-dash hierarchy indicator to be replaced with a custom string.
The separator string is passed through the filter once and then
repeated per hierarchy level.

Renames the internal `$pad` variable to `$separator` for clarity.

Fixes #39106.

Author: vijendrajat

src/wp-admin/includes/class-wp-posts-list-table.php

@@ -1137,7 +1137,16 @@ public function column_title( $post ) {
 			echo '<div class="locked-info"><span class="locked-avatar">' . $locked_avatar . '</span> <span class="locked-text">' . $locked_text . "</span></div>\n";
 		}
 
-		$pad = str_repeat( '&#8212; ', $this->current_level );
+		/**
+		 * Filters the string used to indicate hierarchy level in the posts list table.
+		 *
+		 * The string is repeated once per level, so a child two levels deep will
+		 * have the separator string prepended twice.
+		 *
+		 * @param string  $separator The string used to indicate hierarchy level. Default '&#8212; '.
+		 * @param WP_Post $post      The current post object.
+		 */
+		$separator = str_repeat( apply_filters( 'post_title_child_separator', '&#8212; ', $post ), $this->current_level );
 		echo '<strong>';
 
 		$title = _draft_or_post_title();
@@ -1146,13 +1155,13 @@ public function column_title( $post ) {
 			printf(
 				'<a class="row-title" href="%s">%s%s</a>',
 				get_edit_post_link( $post->ID ),
-				$pad,
+				$separator,
 				$title
 			);
 		} else {
 			printf(
 				'<span>%s%s</span>',
-				$pad,
+				$separator,
 				$title
 			);
 		}

tests/phpunit/tests/admin/wpPostsListTable.php

@@ -309,6 +309,88 @@ public function test_empty_trash_button_should_not_be_shown_if_there_are_no_post
 		$this->assertStringNotContainsString( 'id="delete_all"', $output );
 	}
 
+	/**
+	 * Tests that the default em-dash separator is output for child pages.
+	 *
+	 * @ticket 39106
+	 *
+	 * @covers WP_Posts_List_Table::column_title
+	 */
+	public function test_column_title_uses_default_separator_for_child_pages() {
+		// A child page has post_parent > 0, so column_title() will auto-calculate current_level.
+		$child = self::$children[1][1];
+
+		$this->table->set_hierarchical_display( true );
+
+		ob_start();
+		$this->table->column_title( $child );
+		$output = ob_get_clean();
+
+		$this->assertStringContainsString( '— ', $output );
+	}
+
+	/**
+	 * Tests that the post_title_child_separator filter replaces the separator.
+	 *
+	 * @ticket 39106
+	 *
+	 * @covers WP_Posts_List_Table::column_title
+	 */
+	public function test_post_title_child_separator_filter_changes_separator() {
+		$child = self::$children[1][1];
+
+		$this->table->set_hierarchical_display( true );
+
+		add_filter(
+			'post_title_child_separator',
+			static function () {
+				return '> ';
+			}
+		);
+
+		ob_start();
+		$this->table->column_title( $child );
+		$output = ob_get_clean();
+
+		remove_all_filters( 'post_title_child_separator' );
+
+		$this->assertStringContainsString( '> ', $output );
+		$this->assertStringNotContainsString( '— ', $output );
+	}
+
+	/**
+	 * Tests that the post_title_child_separator filter receives the current WP_Post object.
+	 *
+	 * @ticket 39106
+	 *
+	 * @covers WP_Posts_List_Table::column_title
+	 */
+	public function test_post_title_child_separator_filter_receives_post_object() {
+		$child = self::$children[1][1];
+
+		$this->table->set_hierarchical_display( true );
+
+		$received_post = null;
+		add_filter(
+			'post_title_child_separator',
+			static function ( $separator, $post ) use ( &$received_post ) {
+				$received_post = $post;
+				return $separator;
+			},
+			10,
+			2
+		);
+
+		ob_start();
+		$this->table->column_title( $child );
+		ob_get_clean();
+
+		remove_all_filters( 'post_title_child_separator' );
+
+		$this->assertInstanceOf( WP_Post::class, $received_post );
+		$this->assertSame( $child->ID, $received_post->ID );
+	}
+
 	/**
 	 * @ticket 42066
 	 *