Can't seem to get `:attribute` working with `webc:root`

Here's a simple example of a component where I don't want a wrapping element:

<script webc:setup>
  function featureSize(size) {
    return `feature_${size.replace("/", "-")}`;
  }
</script>
<section webc:root :class="featureSize(size)">
  <!-- ... -->
</section>

This fails with:

[11ty] 1. Having trouble rendering webc template ./src/index.webc (via TemplateContentRenderError)
[11ty] 2. Evaluating a dynamic attribute failed: `:class="featureSize(size)"`.
[11ty] Original error message: featureSize is not a function (via Error)

I'm not sure how to best deal with this. Seems like js isn't accessible on the root element or something like that?

Start by reproducing the example in src/index.webc, focusing on the interaction between webc:setup, webc:root, and the dynamic :class attribute. Trace how the root element evaluates the attribute and determine the expected behavior; this is done when the example renders without the reported featureSize error and the intended class is applied.