[BUG] `npm run` erases script output after the final newline

Is there an existing issue for this?

• I have searched the existing issues

This issue exists in the latest npm version

• I am using the latest npm

Current Behavior

Any script which writes to standard output but does not emit a \n as its final character will have output erased when npm run exits.

Expected Behavior

npm run should never interfere with the output of scripts.

Steps To Reproduce

1. Create a package.json file like this:

   {
     "name": "",
     "version": "0.0.0",
     "scripts": {
       "start": "printf 'this output will vanish'; sleep 3"
     }
   }
   

2. Run npm run start.
3. Observe that the output vanishes after three seconds.

Environment

• npm -v: 11.6.0
• node -v: v22.18.0
• OS Name: macOS 15.6
• System Model Name: MacBook Pro
• npm config:

  //localhost:4873/:_authToken = (protected)
  //registry.npmjs.org/:_authToken = (protected)
  

Start by reproducing the behavior with the package.json script shown in the issue and running npm run start. Trace the npm run output handling to find where the final newline affects displayed output; done means script output without a trailing newline remains visible after the command exits.