Do not set COLUMNS or LINES if already set by user

docs/changelog/2575.bugfix.rst

@@ -0,0 +1,2 @@
+Do not set ``COLUMNS`` or ``LINES`` environment to the current TTY size if already set by the user -
+by :user:`gaborbernat`.

src/tox/execute/local_sub_process/__init__.py

@@ -191,9 +191,9 @@ def __enter__(self) -> ExecuteStatus:
         # adjust sub-process terminal size
         columns, lines = shutil.get_terminal_size(fallback=(-1, -1))
         if columns != -1:  # pragma: no branch
-            self.request.env["COLUMNS"] = str(columns)
-        if columns != -1:  # pragma: no branch
-            self.request.env["LINES"] = str(lines)
+            self.request.env.setdefault("COLUMNS", str(columns))
+        if lines != -1:  # pragma: no branch
+            self.request.env.setdefault("LINES", str(lines))
 
         stdout, stderr = self.get_stream_file_no("stdout"), self.get_stream_file_no("stderr")
         try:

tests/execute/local_subprocess/test_local_subprocess.py

@@ -340,3 +340,21 @@ def _create_shebang_test(tmp_path: Path, env: dict[str, str]) -> tuple[str, Path
     writer = create_autospec(SyncWrite)
     instance = LocalSubProcessExecuteInstance(request, create_autospec(ExecuteOptions), writer, writer)
     return exe, script, instance
+
+
+@pytest.mark.parametrize("key", ["COLUMNS", "ROWS"])
+def test_local_execute_does_not_overwrite(key: str, mocker: MockerFixture) -> None:
+    mocker.patch("shutil.get_terminal_size", return_value=(101, 102))
+    env = dict(os.environ)
+    env[key] = key
+    executor = LocalSubProcessExecutor(colored=False)
+    cmd = [sys.executable, "-c", f"import os; print(os.environ['{key}'], end='')"]
+    request = ExecuteRequest(cmd=cmd, stdin=StdinSource.API, cwd=Path.cwd(), env=env, run_id="")
+    out_err = FakeOutErr()
+    with executor.call(request, show=False, out_err=out_err.out_err, env=MagicMock()) as status:
+        while status.exit_code is None:  # pragma: no branch
+            status.wait()
+    outcome = status.outcome
+
+    assert outcome is not None
+    assert outcome.out == key