This client uses JavaFX (JFXPanel). Java 11 and later do not include JavaFX, so double-clicking the JAR with the system Java runtime can fail with NoClassDefFoundError: javafx/embed/swing/JFXPanel.
After building MesOP040.jar in IDEA, generate the portable release:
.\build-release.ps1
The generated release\MesOP040 directory contains a Java 8 runtime with JavaFX. Distribute the entire directory and start the application with start.bat. Do not start MesOP040.jar directly on the target machine.
If Java 8 is installed in a different location on the build machine:
.\build-release.ps1 -Java8Home "C:\path\to\jdk8"