"I/O operation on closed file" error with CliRunner and echo_via_pager under 8.4

[mschmitzer]

When echo_via_pager is used in the command, CliRunner.invoke fails with:

ValueError: I/O operation on closed file.

This started happening with Click 8.4, does not happen with 8.3.3.

Reproducer:

from click import command, echo_via_pager
from click.testing import CliRunner

@command()
def cli():
    echo_via_pager("Hello, Click!")

runner = CliRunner()
runner.invoke(cli)

Traceback:

Traceback (most recent call last):
  File ".../echo_via_pager.py", line 11, in <module>
    runner.invoke(cli)
    ~~~~~~~~~~~~~^^^^^
  File ".../.venv/echo-via-pager/lib/python3.13/site-packages/click/testing.py", line 676, in invoke
    sys.stdout.flush()
    ~~~~~~~~~~~~~~~~^^
ValueError: I/O operation on closed file.

Environment:

• OS: Ubuntu 24.04.4, Linux 6.17
• Python version: 3.13.5
• Click version: 8.4

[frenzymadness]

I ran into exactly the same issue when I tried to update click in Fedora Linux. 1 test failed, and 189 ended with errors all with the same error: ValueError: I/O operation on closed file..

[anlambert]

Same issue on our side after upgrading click to 8.4, see test output logs.

Looks like a regression introduced by commit c69643b as the new get_pager_file function used in echo_via_pager implementation returns a context manager wrapping a TextIO. So when the wrapped TextIO is sys.stdout, it is automatically closed when exiting the context manager.

[ChrisPappalardo]

Picking up this issue at pycon2026.

[kdeldycke]

For context, this is because #1572 was merged early without tests and the follow up #3405 did not cover this situation.

[kdeldycke]

This fix was not complete. I proposed one at #3533 . @ChrisPappalardo @anlambert @frenzymadness @mschmitzer, can you review and test it?