[css-contain] If a ResizeObserver callback changes the `content-visibility` of an element, should that prevent observations of the same resize event?

[mrobinson]

https://w3c.github.io/csswg-drafts/css-contain-2/#cv-notes states:

From the perspective of a ResizeObserver, the skipped contents of an element never change their size. If these elements become non-skipped later, the resize observation will be delivered if the new size differs from the last size used to notify the resize observer.

A ResizeObsever observation callback might change the content-visibility and force layout of an element though. Consider the situation where that freshly-hidden element also has a ResizeObsever triggered by the same resize event. The question here is whether a web engine should trigger an observation callback for the freshly-hidden element.

To me it seems like the specification implies that immediately after content is hidden it should stop receiving any ResizeObserver callbacks, but I believe that there could be multiple interpretations of the passage above.

cc @emilio

A WPT test demonstrating the situation

```html

x

x

```

[Loirooriol]

This seems somewhat analogous to #6493:

<div id="test">test</div>
<script>
var ro1 = new ResizeObserver(() => {
  console.log("ro1");
  ro2.unobserve(test);
});
ro1.observe(test);
var ro2 = new ResizeObserver(() => {
  console.log("ro2");
});
ro2.observe(test);
</script>

This should log both ro1 and ro2, i.e. the ro2 callback is invoked even if it's no longer observing any element.

So I think that your testcase should be consistent with this, first determine the active observations, then broadcast them regardless of whether some callback does something that would have avoided some observation.

[mrobinson]

This seems somewhat analogous to #6493:

In this case, should the specification be updated to clarify this?