Why is rebuildContainer() required in tests to assert mails correctly?

I agree, we should try to get rid of these unclear $this->rebuildContainer(); calls that seem to be kind of a Drupal Core bug.

Thank you for the documentation. This isn't important and if we find a general core issue, let's postpone this.

To see if this has been fixed upstream already, let's simply remove these calls in the MR here! :) So if the test goes green, it finally works correctly without these calls.

Here are some possibly related core issues, maybe someone can work through them in the future:

General search: https://www.drupal.org/project/issues/drupal?text=rebuildContainer →

• #2066993: Use \Drupal consistently in tests →
• #2252967: rebuildContainer() is not limited to web tests (WebTestBase) →

Reading through the comments in #2066993: Use \Drupal consistently in tests → and the blog entry linked above, $this->contianer might be the root of all evil and this issue: #2066993: Use \Drupal consistently in tests → might be the core issue to solve it?

rebuildContainer() is needed because after the form settings we remain in the same request and the same service container.
It's perfectly fine to use rebuildContainer().

Thank you very much for the helpful information @nevergone - do you have any further information regarding this? A link with details or something? Would be interesting! :)

I can't provide a link, this is how I use it. I included rebuildContainer(), because otherwise, even if you set something in the form, you can't query it elsewhere.