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.

I do think we should mimic the block rendering from \Drupal\block\BlockViewBuilder so that same CSS that works outside of XB works when using blocks in XB. Ideally we should do this before stable but I don't think this needs to block the stable release. Worst case we'd document this in known issues.