Discussion thread: https://mail.openjdk.java.net/pipermail/core-libs-dev/2018-December/057639.html
Currently UnicodeDecoder deals with U+FFFE in the middle of a string as "malformed" as it is a non-character. This has been correct up until Unicode 7. However Unicode 7 includes the corrigendum (http://www.unicode.org/versions/corrigendum9.html) that changed the definition of non-characters. UnicodeDecoder's behavior should be modified to conform to it.