irb seems to be having output buffering issues on Windows.

[YO4]

Environment Information

Provide at least:

>c:\jruby-10.0.2.0\bin\jruby -v
jruby 10.0.2.0 (3.4.2) 2025-08-07 cba6031bd0 OpenJDK 64-Bit Server VM 25.0.1+8-LTS on 25.0.1+8-LTS +indy +jit [x86_64-mswin32]

>ver
Microsoft Windows [Version 10.0.26100.7171]

using irb on windows, nothing is wrote to console when reline interaction, and at last irb dumps whole output when exitting.
This shows output after starting irb and entering RUBY_PLATFORM and ctrl+D.

M:\>c:\jruby-10.0.2.0\bin\irb.bat
WARNING: A restricted method in java.lang.System has been called
WARNING: java.lang.System::load has been called by com.kenai.jffi.internal.StubLoader in an unnamed module
WARNING: Use --enable-native-access=ALL-UNNAMED to avoid a warning for callers in this module
WARNING: Restricted methods will be blocked in a future release unless native access is enabled

=> "java"

▽�[0mirb(main):001> �[0m�[0mR�[0m�[0m�[0m�[97;100mRDoc               �[0m�[37;100m█�[0m�[0m�[0m�[97;100mRELEASE_DATE       �[0m�[37;100m█�[0m�[0m�[0m�[97;100mRUBY_COPYRIGHT     �[0m�[37;100m█�[0m�[0m�[0m�[97;100mRUBY_DESCRIPTION   �[0m�[37;100m█�[0m�[0m�[0m�[97;100mRUBY_ENGINE        �[0m�[37;100m█�[0m�[0m�[0m�[97;100mRUBY_ENGINE_VERSION�[0m�[37;100m█�[0m�[0m�[0m�[97;100mRUBY_PATCHLEVEL    �[0m�[37;100m█�[0m�[0m�[0m�[97;100mRUBY_PLATFORM      �[0m�[37;100m█�[0m�[0m�[0m�[97;100mRUBY_RELEASE_DATE  �[0m�[37;100m█�[0m�[0m�[0m�[97;100mRUBY_REVISION      �[0m�[37;100m▀�[0m�[0m�[0m�[97;100mRUBY_VERSION       �[0m�[37;100m �[0m�[0m�[0m�[97;100mRandom             �[0m�[37;100m �[0m�[0m�[0m�[97;100mRange              �[0m�[37;100m �[0m�[0m�[0m�[97;100mRangeError         �[0m�[37;100m �[0m�[0m�[0m�[97;100mRational           �[0m�[37;100m �[0m�[0mRU�[0m�[0m�[0m�[97;100mRUBY_COPYRIGHT     �[0m�[0m�[0m�[97;100mRUBY_DESCRIPTION   �[0m�[0m�[0m�[97;100mRUBY_ENGINE        �[0m�[0m�[0m�[97;100mRUBY_ENGINE_VERSION�[0m�[0m�[0m�[97;100mRUBY_PATCHLEVEL    �[0m�[0m�[0m�[97;100mRUBY_PLATFORM      �[0m�[0m�[0m�[97;100mRUBY_RELEASE_DATE  �[0m�[0m�[0m�[97;100mRUBY_REVISION      �[0m�[0m�[0m�[97;100mRUBY_VERSION       �[0m�[0mRUB�[0m�[0mRUBY�[0m�[0mRUBY_�[0m�[0mRUBY_P�[0m�[0m�[0m�[97;100mRUBY_PATCHLEVEL�[0m�[0m�[0m�[97;100mRUBY_PLATFORM  �[0m�[0mRUBY_PL�[0m�[0m�[0m�[97;100mRUBY_PLATFORM�[0m�[0mRUBY_PLA�[0m�[0mRUBY_PLAT�[0m�[0mRUBY_PLATF�[0m�[0mRUBY_PLATFO�[0m�[0mRUBY_PLATFOR�[0m�[0mRUBY_PLATFORM�[0mirb(main):001> RUBY_PLATFORM
�[0mirb(main):002> �[0mirb(main):002>

I think this can be resolved by inserting @output.flush before calling the Windows console API.

I remember there were no issues with JRE 8 (with JRuby 9?). But I am not familiar with the changes made to the JVM and JRuby up to this point.
Are the changes in the reline reasonable?
Also, when making changes in reline, should I submit the PR to jruby or reline?

[headius]

Thank you for the report! You should engage with the reline project for your patch. I really appreciate your taking a look into this and I apologize for the delay in responding!

[headius]

FWIW there could be a JRuby bug here in that console IO should perhaps be unbuffered (or less buffered) but let's start the conversation with the reline folks and see what they say.