block__content div is omitted from preview when "Use Experience Builder for page templates in this theme." is enabled

I'm not sure I understand why/how yet, because
\Drupal\experience_builder\Plugin\ExperienceBuilder\ComponentSource\BlockComponent::renderComponent() uses '#theme' => 'block'

Related: 🐛 Placeholders/#lazy_builder is not supported for block component rendering Active + 🐛 Preview is not accurate for the site branding block Active .

Needs debugging to determine the root cause. Thanks for the detailed bug report!

D'oh — no, actually — I misinterpreted!

This is actually similar to 🐛 Enabling XB for managing page template breaks Olivero header Active , and there's nothing we can do about this.

The reason is essentially the same as #3505629-2: [upstream] Olivero's header works only correctly when using the "block" page variant → :

• Drupal core's BlockPageVariant renders the contents of block plugins with an extra wrapper: that's what block.html.twig is about, and that's where <div class="block__content"> is coming from
• XB is rendering all block plugins as-is, without adding more wrappers.

We could avoid this by doing something like core's \Drupal\block\BlockViewBuilder::preRender(), which is the "view builder" for the Block config entity type (which is IRRELEVANT/NON-EXISTENT when using XB!) that is responsible for this, see:

      // Place the $content returned by the block plugin into a 'content' child
      // element, as a way to allow the plugin to have complete control of its
      // properties and rendering (for instance, its own #theme) without
      // conflicting with the properties used above, or alternate ones used by
      // alternate block rendering approaches in contrib (for instance, Panels).
      // However, the use of a child element is an implementation detail of this
      // particular block rendering approach. Semantically, the content returned
      // by the plugin "is the" block, and in particular, #attributes and
      // #contextual_links is information about the *entire* block. Therefore,
      // we must move these properties from $content and merge them into the
      // top-level element.