Drupal CiviCRM Menu not full length (shoreditch and otherwise)
This isn't directly related to the Drupal 8 change but instead is related to a change to the civicrm-drupal7.js file.

The JS is forcing the CiviCRM menu to 97% width - to allow for the Drupal admin toolbar's shortcut bar toggle element. It doesn't however account for sites which don't use the core toolbar module or styling or sites which don't use the toggle element.

A fix for this issue has been committed to CiviCRM (located here) and should be merged in to 5.8 at the latest.

The release summary documentation is worth reading and provides a high level overview of how releases are managed and what kinds of changes are (generally) included/accepted at which stages.