Using `capfd` in fixture breaks output capturing

Consider the following code:

import sys
import subprocess
import pytest

#@pytest.yield_fixture(autouse=True)
#def does_nothing(request, capfd):
#   yield

@pytest.fixture(scope='function')
def emits_stdout():
    sub = subprocess.Popen([
        sys.executable,
        '-c', ';'.join((
            'import time',
            'print("hello world")',
            'time.sleep(0.5)',
            'print("hello whirled")',
        ))
    ])
    yield 'foo'
    sub.wait()

def test_something(emits_stdout):
    import time
    time.sleep(0.7)
    assert False

The output is as expected:

$ python3 -m pytest test_it.py 
==================================== test session starts =====================================
platform linux -- Python 3.5.3, pytest-3.8.0, py-1.6.0, pluggy-0.7.1
rootdir: /home/nikratio/tmp/tests, inifile:
plugins: trio-0.5.0
collected 1 item                                                                             

test_it.py F                                                                           [100%]

========================================== FAILURES ==========================================
_______________________________________ test_something _______________________________________

emits_stdout = 'foo'

    def test_something(emits_stdout):
        import time
        time.sleep(0.7)
>       assert False
E       assert False

test_it.py:27: AssertionError
------------------------------------ Captured stdout call ------------------------------------
hello world
hello whirled
================================== 1 failed in 0.73 seconds ==================================

However, if I uncomment the "noop" fixture, no output is captured anymore:

$ python3 -m pytest test_it.py 
==================================== test session starts =====================================
platform linux -- Python 3.5.3, pytest-3.8.0, py-1.6.0, pluggy-0.7.1
rootdir: /home/nikratio/tmp/tests, inifile:
plugins: trio-0.5.0
collected 1 item                                                                             

test_it.py F                                                                           [100%]

========================================== FAILURES ==========================================
_______________________________________ test_something _______________________________________

emits_stdout = 'foo'

    def test_something(emits_stdout):
        import time
        time.sleep(0.7)
>       assert False
E       assert False

test_it.py:27: AssertionError
================================== 1 failed in 0.75 seconds ==================================

Reproduce the example in test_it.py with and without the no-op fixture using pytest's output-capturing tests and fixture handling as the entry points. Compare the captured output in both runs; done means the subprocess output remains available in the failure report when a fixture requests capfd.