Respect Posts Page (#8259)

* Ensure that Gutenberg is disabled when on the assigned blog page in WordPress (as per similar functionality from edit-form-advanced.php).

* Move code into the gutenberg_can_edit_post() function.

* Use Yoda conditional. Use strict equality, which requires casting the `page_for_posts` option as an integer. Added an inline comment.

* Added unit tests to ensure gutenberg_can_edit_post() returns true when the posts page has content and false when the posts page has no content.

lib/register.php

@@ -269,6 +269,11 @@ function gutenberg_can_edit_post( $post ) {
 		return false;
 	}
 
+	// Disable the editor if on the blog page and there is no content.
+	if ( absint( get_option( 'page_for_posts' ) ) === $post->ID && empty( $post->post_content ) ) {
+		return false;
+	}
+
 	if ( ! gutenberg_can_edit_post_type( $post->post_type ) ) {
 		return false;
 	}

phpunit/class-admin-test.php

@@ -75,6 +75,20 @@ function test_gutenberg_can_edit_post() {
 
 		wp_set_current_user( self::$editor_user_id );
 		$this->assertTrue( gutenberg_can_edit_post( $generic_post_id ) );
+
+		$blog_page_without_content = self::factory()->post->create( array(
+			'post_title'   => 'Blog',
+			'post_content' => '',
+		) );
+		update_option( 'page_for_posts', $blog_page_without_content );
+		$this->assertFalse( gutenberg_can_edit_post( $blog_page_without_content ) );
+
+		$blog_page_with_content = self::factory()->post->create( array(
+			'post_title'   => 'Blog',
+			'post_content' => 'Hello World!',
+		) );
+		update_option( 'page_for_posts', $blog_page_with_content );
+		$this->assertTrue( gutenberg_can_edit_post( $blog_page_with_content ) );
 	}
 
 	/**