Flush sys.stderr when CliRunner finalizes

CHANGES.rst

@@ -6,6 +6,7 @@ Version 8.2.1
 -   Fix flag value handling for flag options with a provided type. :issue:`2894`
     :issue:`2897` :pr:`2930`
 -   Fix shell completion for nested groups. :issue:`2906` :pr:`2907`
+-   Flush ``sys.stderr`` at the end of ``CliRunner.invoke``. :issue:`2682`
 
 Version 8.2.0
 -------------

src/click/testing.py

@@ -510,6 +510,7 @@ def invoke(
                 exc_info = sys.exc_info()
             finally:
                 sys.stdout.flush()
+                sys.stderr.flush()
                 stdout = outstreams[0].getvalue()
                 stderr = outstreams[1].getvalue()
                 output = outstreams[2].getvalue()

tests/test_testing.py

@@ -450,3 +450,24 @@ def test_isolation_stderr_errors():
         click.echo("\udce2", err=True, nl=False)
 
     assert err.getvalue() == b"\\udce2"
+
+
+def test_isolation_flushes_unflushed_stderr():
+    """An un-flushed write to stderr, as with `print(..., file=sys.stderr)`, will end up
+    flushed by the runner at end of invocation.
+    """
+    runner = CliRunner()
+
+    with runner.isolation() as (_, err, _):
+        click.echo("\udce2", err=True, nl=False)
+
+    assert err.getvalue() == b"\\udce2"
+
+    @click.command()
+    def cli():
+        # set end="", flush=False so that it's totally clear that we won't get any
+        # auto-flush behaviors
+        print("gyarados gyarados gyarados", file=sys.stderr, end="", flush=False)
+
+    result = runner.invoke(cli)
+    assert result.stderr == "gyarados gyarados gyarados"