Installing the Varbase Starter site template with drush site:install on plain Drupal CMS fails with "You have requested a synthetic service (kernel)"

Problem/Motivation

On a plain drupal/cms codebase (Drupal core 11.4.6, drupal/canvas 1.11.0, no patches), installing the Varbase Starter site template exits 1 with You have requested a synthetic service ("kernel"), thrown while RecipeAppliedEvent is dispatched. The Drupal CMS browser installer path fails the same way.

Root cause: Drupal\canvas\EventSubscriber\RecipeSubscriber reaches PageVariantMigration::createFromPageRegions(), which installs the canvas_page_template_component module when a theme that has page regions has no page variant yet. Installing a module during that dispatch rebuilds the service container underneath listeners that have not run yet. Drupal CMS's drupal_cms_helper RecipeSubscriber then resolves an OOP hook class through a ClassResolver still holding the pre-rebuild container, and the install dies.

Varbase Starter always meets the trigger condition because it ships canvas.page_region.vartheme_bs5.header and canvas.page_region.vartheme_bs5.footer.

Steps to reproduce

1. Build a plain drupal/cms project (core 11.4.6, drupal/canvas 1.11.0, no patches).
2. Require drupal/varbase_starter.
3. Run drush site:install recipes/varbase_starter.
4. The command exits 1 with You have requested a synthetic service ("kernel").

Proposed resolution

One line in recipe.yml: add canvas_page_template_component to the recipe's own install list, with a short comment and @see pointers to the two upstream classes. The module then already exists when the event fires, so nothing is installed mid-dispatch. Same fix as horizonaid#3622092.

Verified locally (plain drupal/cms, core 11.4.6, canvas 1.11.0, no patches):

• drush site:install recipes/varbase_starter exits 0, 0 warnings, 0 errors.
• The Drupal CMS installer path also exits 0: drush site:install drupal_cms_installer installer_site_template_form.add_ons=varbase_starter.
• Site serves: /, /features, /blog, /about-varbase, /contact-us, /user/login and /search?keywords=varbase all HTTP 200.
• Content present: 14 nodes, 5 Canvas pages, 59 media items, default theme vartheme_bs5.
• No PHP errors in watchdog.

Note for reviewers: the underlying defect is upstream in drupal/canvas — a module installed during RecipeAppliedEvent strands earlier-resolved listeners on a stale container. This change only stops Varbase Starter triggering it. Related upstream report: drupal_cms work item 3591450.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ✅ Testing to ensure no regression
• ➖ Automated unit testing coverage
• ➖ Automated functional testing coverage
• ➖ UX/UI designer responsibilities
• ➖ Readability
• ➖ Accessibility
• ➖ Performance
• ➖ Security
• ➖ Developer Documentation
• ➖ User Guide Documentation
• ❌ Reviewed by human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ➖ Review with the product owner
• ❌ Release notes snippet
• ✅ Release varbase_starter-1.0.2

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Varbase Starter now installs canvas_page_template_component itself, so installing the site template on a plain Drupal CMS codebase no longer fails with You have requested a synthetic service ("kernel").