3.14: branch coverage incorrect with triple-nested context manager

[asottile]

Describe the bug

for some reason a triple-nested context manager seems to confuse branch coverage in python 3.14

To Reproduce

minimal reproduction:

import contextlib


@contextlib.contextmanager
def ctx():
    yield



def f():
    with ctx():
        with ctx():
            with ctx():
                print(1)

f()

with 3.14:

$ bash -xc 'python --version --version && coverage --version && coverage erase && coverage run --branch -m t && coverage report --show-missing'
+ python --version --version
Python 3.14.0b3 (main, Jun 18 2025, 08:59:11) [GCC 13.3.0]
+ coverage --version
Coverage.py, version 7.9.2a0.dev1 with C extension
Full documentation is at https://coverage.readthedocs.io/en/7.9.2a0.dev1
+ coverage erase
+ coverage run --branch -m t
1
+ coverage report --show-missing
Name    Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------
t.py       10      0      2      2    83%   13->14, 13->exit
---------------------------------------------------
TOTAL      10      0      2      2    83%

Expected behavior

3.13 behaviour:

$ bash -xc 'python --version --version && coverage --version && coverage erase && coverage run --branch -m t && coverage report --show-missing'
+ python --version --version
Python 3.13.5 (main, Jun 12 2025, 08:56:14) [GCC 13.3.0]
+ coverage --version
Coverage.py, version 7.9.2a0.dev1 with C extension
Full documentation is at https://coverage.readthedocs.io/en/7.9.2a0.dev1
+ coverage erase
+ coverage run --branch -m t
1
+ coverage report --show-missing
Name    Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------
t.py       10      0      2      0   100%
---------------------------------------------------
TOTAL      10      0      2      0   100%

Additional context

this was tickled by flake8's testsuite -- it works fine with two contexts for some reason though 🤷

[nedbat]

Looks like tracing with sys.monitoring gets it wrong, but sys.settrace gets it right:

% COVERAGE_CORE=ctrace ./py314/bin/coverage run --branch threectx.py
1

% ./py314/bin/coverage report -m
Name          Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------------
threectx.py      10      0      2      0   100%
---------------------------------------------------------
TOTAL            10      0      2      0   100%

% COVERAGE_CORE=sysmon ./py314/bin/coverage run --branch threectx.py
1

% ./py314/bin/coverage report -m
Name          Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------------
threectx.py      10      0      2      2    83%   13->14, 13->exit
---------------------------------------------------------
TOTAL            10      0      2      2    83%

I'll have to investigate...