fix: #3622092 Install canvas_page_template_component from the recipe so Canvas does not install it during RecipeAppliedEvent

Issue: #3622092 Installing the Horizon Aid site template with drush site:install on plain Drupal CMS fails with "You have requested a synthetic service (kernel)"

Problem

On a plain drupal/cms codebase (Drupal core 11.4.6, drupal/canvas 1.11.0, no patches of any kind), installing the Horizon Aid site template fails:

drush site:install recipes/horizonaid

exits 1 with a RuntimeException: You have requested a synthetic service (kernel), thrown while the RecipeAppliedEvent is being dispatched.

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 the listeners that have not run yet. Drupal CMS's own drupal_cms_helper RecipeSubscriber then resolves an OOP hook class through a ClassResolver that is still holding the pre-rebuild container, and the install dies with the synthetic-kernel error.

Horizon Aid always meets the trigger condition because it ships canvas.page_region.vartheme_bs5_horizonaid.header and canvas.page_region.vartheme_bs5_horizonaid.footer.

Control test on the identical codebase: Drupal CMS's own byte site template ships no page regions and installs green — which is what isolates the page regions as the trigger.

The change

Ten lines in recipe.yml only: add canvas_page_template_component to the recipe's own install list, with a comment explaining why and @see pointers to the two upstream classes. The module then already exists when the event fires, so nothing is installed mid-dispatch.

Verification (all run locally today, plain drupal/cms, core 11.4.6, canvas 1.11.0, no patches)

• drush site:install recipes/horizonaid exits 0.
• The Drupal CMS installer path works too: drush site:install drupal_cms_installer installer_site_template_form.add_ons=horizonaid exits 0.
• The site serves: front page plus /about, /resources, /programs, /countries, /events, /donate, /impact, /terms-and-conditions, /user/login and /search all return HTTP 200.
• Search returns results after indexing.
• Content and config present: 70 nodes, 8 Canvas pages, 74 media items, default theme vartheme_bs5_horizonaid.
• Zero PHP errors in watchdog, zero browser console errors.

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 Horizon Aid triggering it. An upstream Canvas issue is worth filing separately.

AI policy disclosure

AI-Generated: Yes

Assisted by Claude Code (Claude Opus 5). The reproduction, the root cause and the fix were verified by a human-run local install, per the Policy on the use of AI when contributing to Drupal.

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