I have no idea how to fix this but I thought I'd note it for the future.
Copy-pasting obscure accented characters (as opposed to accented characters which appear on the ASCII character set) will post properly. Depending on the browser text encoding it will either display correctly or not. However, the post will not be editable - you just get a spinner which will neither time out nor bring back the text.
tested with Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Firefox/52.0
(which is the extended support release, it's fully updated)
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