Transfer steps for OffscreenCanvas arent realm independent

[me-it-is]

What is the issue with the HTML Standard?

Step 8 of the transfer steps tells you to encode a weak reference to a Canvas in a different realm. But in the definition for transfer steps it requires "The resulting data held in dataHolder must be independent of any realm." I don't see how holding a reference to an object in another realm could be realm independent.

[annevk]

See also #10112 on this generally not being well-defined.

We probably have to change the definition of "transfer steps" somehow. SharedArrayBuffer for instance also has to stay within an agent cluster. That's not exactly realm-independent, but also perfectly reasonable.

Perhaps the invariant is that you're either realm-independent or you're agent-cluster-bound.

[annevk]

cc @whatwg/canvas