Add defaultTheme in tests extending BrowserTestBase.

Looks like that test requires classy?

Possibly also use this issue to add core_version_requirement ^8 || ^9 and the same to composer.json as a drupal/core dependency per https://www.drupal.org/node/3070687?

And running tests shows a bunch of deprecations, some like jquery ui might be tricky and several others require 8.8, so the requirement should be ^8.8:

  20x: EntityManagerInterface::getDefinitions() is deprecated in drupal:8.0.0 and is removed from drupal:9.0.0. Use \Drupal\Core\Entity\EntityTypeManager::getDefinitions() instead. See https://www.drupal.org/node/2549139
    2x in EntityDisplayTest::testFieldAccess from Drupal\Tests\field_group\Functional
    2x in EntityDisplayTest::testHtmlElement from Drupal\Tests\field_group\Functional
    2x in EntityDisplayTest::testFieldset from Drupal\Tests\field_group\Functional
    2x in EntityDisplayTest::testTabs from Drupal\Tests\field_group\Functional
    2x in EntityDisplayTest::testAccordion from Drupal\Tests\field_group\Functional
    2x in FieldGroupWithoutFieldUiTest::testLocalActions from Drupal\Tests\field_group\Functional
    2x in ManageDisplayTest::testCreateGroup from Drupal\Tests\field_group\Functional
    2x in ManageDisplayTest::testDeleteGroup from Drupal\Tests\field_group\Functional
    2x in ManageDisplayTest::testNestField from Drupal\Tests\field_group\Functional
    2x in FieldGroupUiTest::testCreateAndEdit from Drupal\Tests\field_group\FunctionalJavascript

  19x: Support for asserting against non-boolean values in ::assertTrue is deprecated in drupal:8.8.0 and is removed from drupal:9.0.0. Use a different assert method, for example, ::assertNotEmpty(). See https://www.drupal.org/node/3082086
    7x in EntityDisplayTest::testAccordion from Drupal\Tests\field_group\Functional
    6x in EntityDisplayTest::testTabs from Drupal\Tests\field_group\Functional
    4x in EntityDisplayTest::testHtmlElement from Drupal\Tests\field_group\Functional
    2x in EntityDisplayTest::testFieldset from Drupal\Tests\field_group\Functional

  4x: entity_get_display() is deprecated in drupal:8.8.0. It will be removed before drupal:9.0.0. Use \Drupal::service('entity_display.repository')->getViewDisplay() instead. See https://www.drupal.org/node/2835616
    4x in MigrateFieldGroupTest::testFieldGroup from Drupal\Tests\field_group_migrate\Kernel\Migrate\d7

  2x: entity_get_form_display() is deprecated in drupal:8.8.0. It will be removed before drupal:9.0.0. Use \Drupal::service('entity_display.repository')->getFormDisplay() instead. See https://www.drupal.org/node/2835616
    2x in MigrateFieldGroupTest::testFieldGroup from Drupal\Tests\field_group_migrate\Kernel\Migrate\d7

  1x: Render #pre_render callbacks must be methods of a class that implements \Drupal\Core\Security\TrustedCallbackInterface or be an anonymous function. The callback was Drupal\field_group\Element\VerticalTabs::preRenderGroup. Support for this callback implementation is deprecated in 8.8.0 and will be removed in Drupal 9.0.0. See https://www.drupal.org/node/2966725
    1x in EntityDisplayTest::testTabs from Drupal\Tests\field_group\Functional

  1x: The "core/jquery.ui.accordion" asset library is deprecated in drupal:8.8.0 and is removed from drupal:9.0.0. See https://www.drupal.org/node/3067969
    1x in EntityDisplayTest::testAccordion from Drupal\Tests\field_group\Functional

Please review the patch.

Still need to change the failing test to use classy instead of stark.

Here I have made changes as suggested in #5

Only the one test that failed with stark (EntityDisplayTest) should be updated to classy, not all of them. That will likely require fewer changes in the future.

Please review the patch.

+++ b/field_group.info.yml
@@ -2,6 +2,6 @@ name: 'Field Group'
 package : Fields
-core: 8.x
+core_version_requirement: ^8 || ^9
 dependencies:
   - drupal:field

Missed this before, but if we go for ^8 then we should keep core: 8.x so that earlier core versions can also still install it.

Now it looks good.

Thx for the patches and the reviews. I committed it to dev.