Investigate using :has-slotted when it's available in all browsers

[jattasNI]

🧹 Tech Debt

We have some places where we apply styles based on whether a slot contains content. Our current pattern for doing this involves using the FAST slotted directive to run JS code that tracks content being added and removed, then set classes on the component based on that state.

The native :has-slotted CSS pseudoclass is meant to provide the same capability using pure CSS. It's not currently available in Safari but once it is we should investigate whether we can use it instead to simplify our code and reduce execution overhead.

A non-exhaustive list of places that do this:

• chat message footer-actions slot
• dialog footer slot
• ni/FAST start/end slot pattern

Lots more context on this CSSWG issue, including a tangent about why :has isn't appropriate since it doesn't operate across the shadow root boundary: w3c/csswg-drafts#6867