Warnings logged for missing field block plugins when enabling layout builder on an entity view display for the first time

This might not be only entity view displays, but any entity using layout builder.

See 🐛 [warning] The "system_menu_block:welcome" block plugin was not found Active . Easiest way to reproduce is ddev drush si --yes demo_umami

Given this will affect Drupal CMS until we switch to XB and it makes the system look buggy, can we figure out a way to try to suppress these somehow soon? Even a hacky way?

🐛 Block plugin not found warnings on install Active

I wrote a test for this and found the cause -- the issue is the parent::preSave() call in \Drupal\layout_builder\Entity\LayoutBuilderEntityViewDisplay::preSave(), which invokes some inline block related hooks that expect all the block plugins in the section list to be instantiate-able, but the field block definition doesn't exist yet because apparently the block cache hasn't been cleared.

Except that it has. LB does clear the block plugin cache when a field is created (in \Drupal\layout_builder\Hook\LayoutBuilderHooks::fieldConfigInsert()). In my test, I am very explicitly doing that. You'd expect that would cause the block to become available right away, but no, because \Drupal\layout_builder\Plugin\Derivative\FieldBlockDeriver::getFieldMap() is only exposing fields used by entity view displays that have layout. The problem is...the view display hasn't been saved yet: We're still in the middle of pre-saving it!

So I think that's your problem right there. I'm not sure what the correct solution is here because is some very sensitive logic. But at least we have a test now so we'll know we fixed it.

I am adding the STR from Comment #3 to the issue summary. I have noticed the same thing.