Version: 56
OS: Android

What steps will reproduce the problem?

const e = new BeforeInstallPromptEvent("", {
    cancelable: false
});
console.assert(e.cancelable === false);

Source: https://github.com/w3c/manifest/pull/506/files

What is the expected output?
Assert succeeds (e is not cancelable).

What do you see instead?
Assert fails (e is still cancelable by default).