Extensions bug in Safari Technology Preview 141

I just want to notify everyone of a bad bug in Safari Technology Preview 141 that affects Safari web extensions and Safari app extensions. The bug is that JavaScript instanceof returns the wrong value in MutationObserver callbacks. I suspect that a lot of extension developers use MutationObserver, so make sure to test in Safari Technology Preview 141 to see whether you're affected. The bug does not exist in Safari 15.4, released yesterday. Hopefully we can stop this bug from shipping in regular Safari, or it could be a giant mess for extension developers!

WebKit bug link