In case someone else is also confused by this, it seems that instanceof checks whether the objects prototype matches, and these prototypes are not shared across different contexts, which iframes are [0]. (Though I would still like to know why it works like this.)

[0] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

No, it was not given a Promise. It was given a foreign object from another window. If you want to inspect another window you should not be reusing code that is designed for single threaded operations. Instead, have a layer that translates, serializes, or explicitly defines an interface that the objects we are dealing with are foreign and need to be transformed. Then the abstraction implementation details of dealing with multiple windows become a concern of a single layer and not your entire codebase. Implicitly and magically treating a foreign window as this window, will fail in many subtle and unknown ways. The "brokenness" you mention is not in that implementation, it is correctly breaking, telling you that what you are doing is wrong, then you try to bypass the error instead of fixing your approach.

Though, let's also appreciate just how niche that case is. I'd be surprised if more than 0.5% of the JS devs reading this will ever encounter that scenario where they are reaching across VMs like that in their life.

`obj instanceof Promise` and `typeof obj.then === 'function'` (is-promise) are much different checks. Frankly, I don't think either belongs in a library. You should just write that code yourself and ponder the trade-offs. Do you really just want to check if an object has a then() method or do you want to check its prototype chain?