Currently if you run the following test:

// |reftest| shell-option(--enable-shadow-realms) skip-if(!xulRuntime.shell)

var realm = new ShadowRealm();

function innermost() {
    assertEq(/onion/.test(realm.evaluate("new Error().stack")), false);
}

function onion1() {
    innermost();
}

function onion2() {
    onion1();
}

onion2();

if (typeof reportCompare === 'function')
    reportCompare(true, true);

The test will fail, as the stack returned from the evaluate call is the whole execution stack:

@js/src/tests/non262/ShadowRealms/errors.js:6:1
innermost@js/src/tests/non262/ShadowRealms/errors.js:6:17
onion1@js/src/tests/non262/ShadowRealms/errors.js:10:5
onion2@js/src/tests/non262/ShadowRealms/errors.js:14:5
@js/src/tests/non262/ShadowRealms/errors.js:17:1

We may wish to actually censor the stacks from inside the shadow realm to stop at the evaluate call.

(Also interesting: What is the filename reported by a new error inside of the shadow realm?)