Bug: Props in child component defined in mixin are missing with shallowMount on @vue/compat

[cody-collins]

Describe the bug
With shallowMount on Vue 3 + @vue/compat, properties of a child component are undefined when using them through a mixin.

If the property is moved from the mixin directly to the component, then the property is defined.

Also if the shallowMount is switched to mount, the property is defined.

To Reproduce

I created a demo repo where this issue can be reproduced, with tags for various working and not working states. It includes the following important files:

• Parent Component: App.vue
• Child Component: HelloWorld.vue
• Mixin: HelloWorldMixin.js
• Test: App.test.js

git clone https://github.com/cody-collins/vue-compat-mixin-demo.git
cd vue-compat-mixin-demo
npm install
npm test

And the following tags:

• Working: Vue 2 + VTU 1
• Broken: Vue 3 + VTU 2
• Working: Vue 3 + VTU 2 + mount instead of shallowMount
• Working: Vue 3 + VTU 2 + props moved to component

Expected behavior
The second assertion should pass in the broken case

[cexbrayat]

Hi @cody-collins

Thanks for the repros. compat mode is the realm of @xanf , so let's ping him

[lmiller1990]

I do not doubt this is a bug, but I suspect fixing this will be quite complex. It may be a good time to attempt to move your test to use mount, which is a lot less complexity and more in parity with both Test Utils v1 and production in general.