Why does this spec use dictionaries containing FrozenArray<>s, instead of sequence<>s?

[domenic]

We're hoping to remove support for dictionaries containing frozen arrays, in whatwg/webidl#1399. It's not clear why this spec makes the array objects immutable, when the dictionaries themselves are (like all dictionaries) mutable.

[reillyeon]

I recall some old guidance around using frozen arrays in return values. It might've been misapplied here.

Is there any compat risk in changing this?

[inexorabletash]

The IDL issue calls out the plausible use case of "The exception is if some of these APIs are caching the FrozenArray<>s to avoid creating new JS Array values each time, for performance reasons." - a quick glance at the Chromium impl it doesn't look like we're caching.

The compat risk should be minimal - it would depend on a caller receiving the array via the output dictionary, and relying on it being immutable. Not impossible, but unlikely.