Brent accidentally sent me down a wormhole that led me here. In short, we should match the spec as of https://github.com/whatwg/html/pull/4734; this means (tl;dr): SharedArrayBuffer is available, though the constructor is not, regardless of COOP+COEP, but cannot be sent via postMessage. With suitable restrictive COOP+COEP headers, the SharedArrayBuffer constructor is available and can be sent via postMessage. See https://wpt.fyi/results/html/infrastructure/safe-passing-of-structured-data?label=master&label=experimental&product=chrome&product=firefox&product=safari&aligned This also explains Bug 220364.
<rdar://problem/96314637>
(In reply to Sam Sneddon [:gsnedders] from comment #0) > Brent accidentally sent me down a wormhole that led me here. Specifically, it led to me writing https://github.com/web-platform-tests/wpt/pull/34678 because they don't pass with our current restrictions (as it turns out, that change is also wrong because I didn't make them load over HTTPS, but that's a problem for next week).
Fixed in bug 243764 *** This bug has been marked as a duplicate of bug 243764 ***