make the 'group' property on EntityReferenceSelection plugins implicitly derived

Can we still reach out to whoever wrote this to hear why the whole concept of groups was introduced to begin with? I feel like another solution might be to just drop the whole idea of groups.

In theory we could do that. The field settings would instead let you pick all the plugins that declare they work with the current entity type.

But in the SelectionPluginManager there's a lot of logic for selecting the 'best' plugin, so of which seems irrelevant given what EntityReferenceItem does, but which could be in use in the wild. In particular, someone could have added a default:node_extra plugin and relied in the weight to get that selected automatically in a custom form that uses the entity_autocomplete element. There'd be a lot of behaviour to first DOCUMENT (!!!!) then write tests for (there are none!! SURPRISE!!!) and then deprecate with BC. I'm not against doing that, but the MR here is a nice quick solution which means no more headdesking for devs trying to write a plugin.

The Needs Review Queue Bot → tested this issue. It fails the Drupal core commit checks. Therefore, this issue status is now "Needs work".

This does not mean that the patch necessarily needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

Consult the Drupal Contributor Guide → to find step-by-step guides for working with issues.

Something not mentioned in the summary is that there can be more than one colon. The ID could be foo:bar:baz

I'm pretty sure that when it comes to selection handlers, there can be multiple colons.

True, but that won't make any difference. It's the base plugin ID that has to match the group.