>I do not agree that slice() should operate on extended grapheme clusters. Don’t lump the grapheme cluster/scalar value split in with the sins of UTF-16 and its unreliable code point/code unit split.

Yeah, I think that's fair. I didn't really think this through as I was writing it.

I'm not even so sure "ending up with nonsense" here is the worst outcome. It might be unavoidable with this approach and if that had been the only problem this bug might have been less memorable.

The real problem—which I mention didn't articulate/emphasize particularly well—was that these invalid surrogate pairs were getting passed into `encodeURIComponent` somewhere deep in the stack and choking catastrophically on them. That was the "real" bug at the end of the day, but the invalid surrogate pairs and the way they were getting created on the way were a fun journey to untangle.