Sorry, rather a question than an issue.
Looked at the format of asciinema v2 which looks to be utf8-unicoded.
"it's UTF-8 encoded JSON string, with all non-printable Unicode codepoints encoded as \uXXXX"
Looked thru the code to figure out how this is handled and passed to xterm.write that seems to expect utf16.
If you could point out where this is dealt with I would really appreciate it!