When launching JShell programmatically (i.e., from a Java program, using the JavaShellToolBuilder interface, instead of the command line) for an interactive session, it's not currently possible to inform JShell what the terminal window's dimensions are.

As a result, JShell defaults to 80x24 and line editing becomes almost impossible because of the scrambled screen contents.

Suggestion: add a new method JavaShellToolBuilder.windowSize() which allows providing a more informed "hint" to JShell what the actual screen dimensions are.