Pass original structure position to usort.

includes/class-sensei-course-structure.php

@@ -774,14 +774,18 @@ private function validate_item_structure( array $raw_item ) {
 	public static function sort_structure( $structure, $order, $type ) {
 		if ( ! empty( $order )
 		&& [ 0 ] !== $order ) {
+			// Remember current position in structure.
+			foreach ( $structure as $key => $value ) {
+				$structure[ $key ]['position'] = intval( $key );
+			}
 			usort(
 				$structure,
 				function( $a, $b ) use ( $order, $type ) {
 					// One of the types is not being sorted.
 					if ( $type !== $a['type'] || $type !== $b['type'] ) {
 						// If types are equal, keep in the current positions.
 						if ( $a['type'] === $b['type'] ) {
-							return 0;
+							return $a['position'] === $b['position'] ? 0 : ( $a['position'] > $b['position'] ? 1 : -1 );
 						}
 
 						// Always keep the modules before the lessons.
@@ -793,7 +797,7 @@ function( $a, $b ) use ( $order, $type ) {
 
 					// If both weren't sorted, keep the current positions.
 					if ( false === $a_position && false === $b_position ) {
-						return 0;
+						return $a['position'] === $b['position'] ? 0 : ( $a['position'] > $b['position'] ? 1 : -1 );
 					}
 
 					// Keep not sorted items in the end.
@@ -804,6 +808,10 @@ function( $a, $b ) use ( $order, $type ) {
 					return false === $b_position || $a_position < $b_position ? -1 : 1;
 				}
 			);
+			// Forget previous positions in structure.
+			foreach ( $structure as $key => $value ) {
+				unset( $structure[ $key ]['position'] );
+			}
 		}
 		return $structure;
 	}