When I used Surrogate Pair character on JTextField on macOS, unexpected symbol (square) was displayed at the end of line.
I used following build.
$ ~/jdk-16.jdk/Contents/Home/bin/java -version
openjdk version "16-ea" 2021-03-16
OpenJDK Runtime Environment (build 16-ea+24-1553)
OpenJDK 64-Bit Server VM (build 16-ea+24-1553, mixed mode, sharing)
Please see attached screen shot and testcase.
According to my investigation, I could recreate this issue after JDK13.
I could not recreate this issue on JDK 12.0.1, but I could recreate it on JDK 12.0.2.
I assume this issue came from HarfBuzz upgrade (JDK-8220392)
Base update might be JDK-8210782.
In these days, HarfBuzz was upgrade by JDK-8247872, it should be in 16-b24.