[csswg-drafts] [resize-observer] observe() not triggering initial callback (#5529)

trusktr has just created a new issue for https://github.com/w3c/csswg-drafts:

== [resize-observer] observe() not triggering initial callback ==
Has this more recently changed to where `new ResizeObserver(...).observe(el)` no longer triggers an initial observation? Or do I experience a browser bug?

Running this in Chrome console does not show any output:

```js
new ResizeObserver(() => console.log('change')).observe(document.body)
```

If we don't get the initial firing of the callback, how do we manually calculate the `borderBoxSize` and `contentBoxSize` arrays? It seems fairly complicated to get come up with that data (and that's why it's nice to receive it in the callback).

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/5529 using your GitHub account


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