1.34.0: test_environment_variables fails on really fast CI runners

Step 1: Provide a summary of your problem

I'm the Debian maintainer for tmuxp, and on fast amd64 and s390x CI hosts, test_environment_variables sometimes fails. This happens in different locations, but always on a line that asserts pane.capture_pane.

This is Debian bug https://bugs.debian.org/1060775

Step 2: Provide tmuxp details

-------------------------
environment:
        dist: Linux-6.6.8-amd64-x86_64-with-glibc2.37
        arch: x86_64
        uname: Linux; hulk; 6.6.8-amd64
        version: #1 SMP PREEMPT_DYNAMIC Debian 6.6.8-1 (2023-12-22)
-------------------------
python version: 3.11.7 (main, Dec  8 2023, 14:22:46) [GCC 13.2.0]
system PATH: /usr/local/bin:/usr/local/sbin:/bin:/usr/bin:/sbin:/usr/sbin
tmux version: 3.3
libtmux version: 0.25.0
tmuxp version: 1.34.0
tmux path: /bin/tmux
tmuxp path: /usr/lib/python3/dist-packages/tmuxp

Step 3: Describe the problem:

I've tried raising the timeout in that test from 0.3s to 3s but we still see failures:

https://ci.debian.net/packages/t/tmuxp/testing/s390x/41861855/

Steps to reproduce:

I can't reproduce this on my personal development machine, unfortunately.

Observed Results:

Here's the failure log:

=== python3.11 ===
============================= test session starts ==============================
platform linux -- Python 3.11.7, pytest-7.4.3, pluggy-1.3.0
rootdir: /tmp/autopkgtest-lxc.1o9ufl7q/downtmp/autopkgtest_tmp
plugins: libtmux-0.25.0, rerunfailures-12.0, hypothesis-6.92.2, mock-3.12.0
collected 194 items / 2 deselected / 192 selected

tests/test_plugin.py ..........                                          [  5%]
tests/test_shell.py ..                                                   [  6%]
tests/test_util.py .......                                               [  9%]
tests/cli/test_cli.py ........                                           [ 14%]
tests/cli/test_convert.py ........                                       [ 18%]
tests/cli/test_debug_info.py .                                           [ 18%]
tests/cli/test_freeze.py ......                                          [ 21%]
tests/cli/test_import.py .......                                         [ 25%]
tests/cli/test_load.py ...............s........                          [ 38%]
tests/cli/test_ls.py .                                                   [ 38%]
tests/cli/test_shell.py ....................                             [ 48%]
tests/tests/test_helpers.py ..                                           [ 50%]
tests/workspace/test_builder.py ...s......Fs............................ [ 70%]
..........                                                               [ 76%]
tests/workspace/test_config.py ..............                            [ 83%]
tests/workspace/test_finder.py ..................                        [ 92%]
tests/workspace/test_freezer.py ...                                      [ 94%]
tests/workspace/test_import_teamocil.py ........                         [ 98%]
tests/workspace/test_import_tmuxinator.py ...                            [100%]

=================================== FAILURES ===================================
__________________________ test_environment_variables __________________________

session = Session($1 libtmux_m6iuy9kl)

    @pytest.mark.skipif(
        has_lt_version("3.0"),
        reason="needs -e flag for new-window and split-window introduced in tmux 3.0",
    )
    def test_environment_variables(
        session: Session,
    ) -> None:
        """Test setting of environmental variables in tmux via workspace builder."""
        workspace = ConfigReader._from_file(
            test_utils.get_workspace_file("workspace/builder/environment_vars.yaml")
        )
        workspace = loader.expand(workspace)
    
        builder = WorkspaceBuilder(session_config=workspace, server=session.server)
        builder.build(session)
        # Give slow shells some time to settle as otherwise tests might fail.
        # Debian: raise timeout from 0.3s to 3s to workaround flakyness on really
        # fast Debian CI workers (see https://bugs.debian.org/1060775)
        time.sleep(3.0)
    
        assert session.getenv("FOO") == "SESSION"
        assert session.getenv("PATH") == "/tmp"
    
        no_overrides_win = session.windows[0]
        pane = no_overrides_win.panes[0]
        pane.send_keys("echo $FOO")
>       assert pane.capture_pane()[1] == "SESSION"
E       IndexError: list index out of range

tests/workspace/test_builder.py:410: IndexError

Here's a couple of other failed runs, prior to changing the timeout (some of them happened with tmuxp 1.31, as the problem has been present for a while):

https://ci.debian.net/packages/t/tmuxp/testing/amd64/40953851/
https://ci.debian.net/packages/t/tmuxp/testing/amd64/40904252/
https://ci.debian.net/packages/t/tmuxp/testing/s390x/41855336/
https://ci.debian.net/packages/t/tmuxp/testing/s390x/41828952/
https://ci.debian.net/packages/t/tmuxp/testing/s390x/41730238/

Expected Results:

test_environment_variables reliably passes.

Start with tests/workspace/test_builder.py and test_environment_variables, especially the pane.capture_pane() assertion at line 410. Run this test under the reported CI conditions and compare the existing timeout behavior. Done means the test reliably passes on fast amd64 and s390x CI runners without intermittent IndexError failures.