Add helper function to check if post is current or descendent.

includes/functions/theme-functions.php

@@ -457,3 +457,29 @@ function trestle_image_has_size( $image_id, $image_size = null ) {
 	return false;
 
 }
+
+/**
+ * Check if post is, or is a child of, a target post.
+ *
+ * @since 1.0.0
+ *
+ * @param string $post_id   Post ID to check.
+ * @param string $target_id Target post ID to check against.
+ *
+ * @return true|false
+ */
+function trestle_is_current_or_descendant_post( $post_id = '', $target_id = '' ) {
+
+    // If the current post is the target post, return true.
+    if ( $post_id == $target_id ) {
+        return true;
+    }
+
+    // If the current post is a descendant of the target post.
+    if ( in_array( $target_id, get_post_ancestors( $post_id ) ) ) {
+        return true;
+    }
+
+    return false;
+
+}