Extending Single File Component results to double mounting

[itomtom]

Version

2.6.10

Reproduction link

https://codesandbox.io/s/vue-template-rr65f

Steps to reproduce

Run codesandbox and you will see 2 'Created Parent' and 1 'Create Child' console logs.

What is expected?

Only 1 'Create Child' console log

What is actually happening?

Extending the parent template and class is causing extra mounts not required.

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What is the correct way for extending Single File Components to reuse template and class?

[posva]

I don't know what you are trying to do by extending the components like that because the template cannot be reused but when it comes to created hooks (as others), they are merged to be both kept (https://vuejs.org/v2/guide/mixins.html#Option-Merging). But basically, only one of them should have a template as only the last one will be used: https://codesandbox.io/s/vue-template-6w60s

To reuse template parts, you should use scoped slots

See #6811 and #5401