You are right, this is related to Vue's implementation details. I am a little confused about "both forms of rendering to behave the same way" - is wrapper.html() different?

I am not sure if there is a problem or bug here.

@lmiller1990 - Thanks for at least confirming I'm not crazy.

Regarding your comment on my remark:

You can image that my component has some logic that does things on the $attrs object, the content of $attrs is different depending on how I render the component.

In one instance my component has access to $attrs.class, and on the other it does not.

I'm fine with one of the two being right, but it's a bit confusing when they behave differently.

This won't just be a problem in VTU though right - this is coming from Vue core.

Do you have an example test where this is a problem?

I think this is problem unique to VTU and unrelated to Vue(kind of)

Vue treats class and style as a special case, but VTU does not.
I think this inconsistency is a problem, as at the very least it's unexpected.

In our test case, we have a component that passes down all the $attrs it receives to a child component.

On top of all other $attrs we expected, both style and class were also cascaded down to the inner html, but this did not happen when we rendered with Vue.

It took some fiddling to find out that it's the way we write our tests that creates this effect.
If we run our tests in a template, everything works as expected.

rendering the component

   mount(MyComp, {
      propsData: {
         class='abc'
      },
    });

would result in this:

    <span class="abc"><span>
</div>

however this form:

mount(
      {
        template: `
          <my-comp
            class='abc'
          />
        \`
      },
      {
        components: { MyComp }
      }
    );

would result in this output

<div class="abc">
    <span><span>
</div>

Create a minimal repro, where we can assert what exactly you are doing. I think it may be a mix of vtu and user error

No problem,

Repro:
https://github.com/IamTheHttp/vtu-issue-1579-repro

Example component
https://github.com/IamTheHttp/vtu-issue-1579-repro/blob/master/src/components/HelloWorld.vue

Test cases (That demonstrates the problem)
https://github.com/IamTheHttp/vtu-issue-1579-repro/blob/master/tests/unit/example.spec.js

@dobromir-hristov Is there any chance to confirm whether it's a user error or a VTU issue?

Had a think about this. I think the current behavior is correct. According to the Vue docs, $attrs does not include class and style, so that is why it is not appearing in the wrapperOfTemplateComponent example.

Let me know if I am missing something and we can reopen this.

@lmiller1990 my only concern, as a developer and tester, is that the two cases are not identical.

Now that I know that there's some difference between the two, I will always use the option closest to Vue.

Our test was working perfectly using mount(MyComp) but failing in production, as a person who loves automation and trusts it, this made me feel extremely uneasy. (If we can't trust our tests, what can we trust?)

this error was spotted by chance, and we could've missed it.

I didn’t realise this was working differently in prod. Maybe we need to investigate more.