fix(nuxt): prefer using currentInstance if available in useNuxtApp

[huang-julien]

🔗 Linked issue

resolve #19623

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

hi 👋 this is a workaround for #19623

when using useNuxtApp there is a small risk of CRSP with sync server components. However this risk gets higher with async components.
This keeps the current implementation,_ it just prefer using the current instance of the component if it is available.

Needed for #19605

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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[pi0]

👍🏼

[pi0]

This seems a nice fix and PR. Only (shared by @danielroe) it seems getCurrentInstance() might suffer from some context leaks as well vuejs/core#7743.

We might try other ideas to mitigate issues with component islands such as using a variable unctx name for island mode

[huang-julien]

So... shall I close this PR then ? 🤔
Hoever i'm not very familiar with the usage of unctx.
Tried to implement some tests in #19605 but even fixtures tests are falling into this issue (due to async components)😅 ...

[huang-julien]

closing as solved by #19753