I upgraded three sites today.
Site 1 - Already had Civi 5.6 - updated to Drupal 7.60 and Shoreditch (SD) Alpha26. Result: CiviCRM menu is not full length.
Site 2: Updated from SD 25 to 26 - and to Drupal 7.60 - No issues - Was civi 5.3, upgraded to 5.6 - Result: Civi menu is not full length.
Site 3: Updated to Civi 5.6 and SD 26 - Wordpress site - no issues.
I do not think this is a Shoreditch issue, rather there is a menu issue between Civi 5.6 & Drupal 7.6.
Could this be related to this Drupal 8 change?