No, NUnit supports this both from the CLI and within IDE (I tested Rider and VS 2022), see here and here.

I assume that NUnit is somehow able to detect whether a GUI test run is triggered from a single test, a test class or something like Run all tests. For the first two cases, NUnit can check whether the test respectively the test class is marked with [Explicit]. If so, the test respectively all tests from the test class can be executed.

So I think it boils down to the question: is xUnit able to detect on which set of tests (single test, test class or all tests) is started?

The two APIs are "run this set of tests" and "run all tests". There is no "run this test class" gesture via the API. So I presume that NUnit shows the test in Test Explorer as runnable, but unless it's explicitly asked for, it doesn't actually ever run?

Please see the following screenshot:

You're absolutely right with you assumption.

Marking as resolved because of issue #2518. There are still some open questions to be resolved there, please pick up further design discussion in the issue.