Test placeholders with lazy builder

So, with Drupal 10.1.0, I tried to edit the core/modules/big_pipe/templates/big-pipe-interface-preview.html.twig template to see the result.

And the problem is that Bootstrap placeholder expects the loading stuff to already take the space on the page while Drupal lazy builder render stuff after, it does not reserve the space on the page.

Also by default the "preview" variable is empty in the template.

Like you will see on the screen capture, this would add a lot of flickering to a lot of blocks.

So let's do nothing and people who want to have a placeholder will have to add some markup using #lazy_builder_preview like mentioned in the change record.