Then I should be on an admin page

Assert the current page is, or is not, one of the site's administration pages. Core flags its admin routes and says so in drupalSettings.path.currentPathIsAdmin, whatever the theme; that flag, not the URL prefix, is what decides whether the administration theme is used. Where a page carries no settings, the path-admin body class of core's themes (the "admin page" named selector) and then the /admin URL prefix decide.

Examples

Then I should be on an admin page
And we should be on an administration page
Then should be on an admin page
And I should not be on an admin page
Then we should not be on an administration page

Pattern

Then(/^(I |we )*should( not)* be on an admin(?:istration)? page$/, …)

Part of @webship-js/drupal-cms-steps (github.com/webship/drupal-cms-steps), a step pack for Webship-js 2.0.x.