1. Install poetry
2. Install asdf
3. Run make install

The bug can be reproduced by running make, which:

1. Builds a Phoenix application (mix is one of the processes sensible to this bug, but I have seen it happen with nodemon too)
2. Starts the Phoenix server
3. Retrieves the server /index.html page every second

To trigger the bug, hover the mouse over the Textualize app, you should immediately see timeouts in the output. If you look at the process with htop, you also see its state go from S (sleeping) to T (traced/stopped)

The bug is introduced by this change (everything works fine witouth piping + os.setpgrp):

diff --git a/bug/cli.py b/bug/cli.py
index bd6619a..41bb82b 100644
--- a/bug/cli.py
+++ b/bug/cli.py
@@ -1,3 +1,4 @@
+import os
 from typing import Callable, Any
 import asyncio
 
@@ -12,7 +13,11 @@
 
 async def run_process(*args, print_function: PrintFunction, cwd=None):
     proc = await asyncio.create_subprocess_exec(
-        *args, stdout=asyncio.subprocess.PIPE, cwd=cwd
+        *args,
+        stdout=asyncio.subprocess.PIPE,
+        stderr=asyncio.subprocess.STDOUT,
+        cwd=cwd,
+        preexec_fn=os.setpgrp,
     )
     if proc.stdout is not None:
         async for line in proc.stdout:

You can also run the non-gui version to check the issue only happens when running under Textualize.