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

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/Reference/Operators/instanceof#Description https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...