JRuby issues with irb and reline

Description

I have been working to switch JRuby to reline+irb gems and things are close but I have seen two issues testing it today.

1. Backspace does not work. It seems to be ignored. ^H works, but shows the ^H character briefly before backspacing.
2. The prompt line seems to print out again before running whatever code you entered:

$ irb
irb(main):001:0> puts
irb(main):001:0> puts

=> nil
irb(main):002:0> exit
irb(main):002:0> exit

I am not sure if these are IRB issues or Reline issues, and I need help figuring out what is wrong!

Note: JRuby uses an FFI-based version of io-console that we recently upstreamed to the gem. The io-console tests can't run green on JRuby due to our broken PTY library, but they run green on CRuby. There may still be bugs. See https://github.com/ruby/io-console/pull/22 for the work I did to get it working.

To investigate this it would be best to use the irb+reline branch I have pushed here: https://github.com/jruby/jruby/pull/5803

Note another issue we thought might be fixed by this move still seems broken under new IRB + reline: https://github.com/jruby/jruby/issues/1372

Result of irb_info

irb(main):001:0> irb_info
=> 
Ruby version: 2.6.5
IRB version: irb 1.3.4 (2021-02-25)
InputMethod: ReidlineInputMethod with Reline 0.2.4
RUBY_PLATFORM: java

Terminal Emulator

I am using MacOS Terminal.

Setting Files

No setting files.

Use the irb+reline branch from https://github.com/jruby/jruby/pull/5803 and reproduce the backspace and repeated-prompt behavior on JRuby. Compare IRB, Reline, and the FFI-based io-console path, starting with the io-console tests and the referenced JRuby issue; done means identifying the responsible component and covering both reported behaviors.