Attach ResizeObserver when component is mounted

[dhruvkb]

Fixes

Fixes #171 by @obulat

Description

This PR defers the initialisation of the observer variable to the onMounted hook which runs on the client where the ResizeObserver API is available.

Technical details

The ResizeObserver API is provided by window, and thus only exists on the client. Using it in the setup() function causes it to be run on the server side of SSR leading to an error. Marking the component as <ClientOnly> is not an elegant solution.

Tests

See reproduction steps on the issue.

Checklist

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• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.

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[zackkrida]

Great! It works 😄