Grouping support for Item and Email FormElements

Was previously tagged for tests in #6 which still needs to happen please.

I also can confirm that the patch provided in #5 fixes the bug.
My setup:
Drupal 10.2.4
Field Group 8.x-3.4
Account field split 2.0.2

So as the issue only seems to occur when grouping elements using the field_group module, do we really want to rebuild the logic that field_group uses for grouping elements into a core test? Creating a dependency to the field_group module for a specific core test doe also not feel right.

I've created a MR which contains a test which checks if all FormElement plugins defined by core have a #process callback to processGroup, which solves the problem. Running that test, I discovered additional FormElements which also did not have the #process callback.

Probably work in this MR will be removed again when https://www.drupal.org/project/drupal/issues/2190333 📌 Make #group FAPI / render feature work on all form/render #types out of the box Needs work gets fixed.

Issue summary needs to be updated. I added the template but left the sections empty. Main ones needed are steps to reproduce and proposed solution. If no UI or date model change NA is fine.

After that probably can ask sub-maintainer for review of solution.

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.

I don't think anything needs to be changed? The mentioned class (Drupal\Core\Render\Element\FormElementBase) by the needs-review-queue-bot does not exist and FormElement is not marked as deprecated?

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.

patches don't work for drupal 10.2.7 :(

Patch version for drupal 10.4.0

Rebased onto latest 11.x branch.

Multiple tests are failing now, but I haven't managed to fix them. RenderElementBase::preRenderGroup and RenderElementBase::preRenderGroup seem to do something that makes some elements don't appear on the page on functional tests.