Fix echo_via_pager closing sys.stdout when used with CliRunner

src/click/_termui_impl.py

@@ -449,6 +449,12 @@ def get_pager_file(color: bool | None = None) -> t.Generator[t.TextIO, None, Non
             yield stream
         finally:
             stream.flush()
+            # MaybeStripAnsi wraps an existing buffer (e.g. sys.stdout.buffer).
+            # io.TextIOWrapper takes ownership of the buffer and closes it on
+            # GC, which would silently close sys.stdout. Detach first so the
+            # buffer is not closed when this wrapper is collected.
+            if isinstance(stream, MaybeStripAnsi):
+                stream.detach()
 
 
 @contextlib.contextmanager

tests/test_testing.py

@@ -747,3 +747,16 @@ def cli() -> None:
     # Click output is not leaking into Pytest.
     assert "inside-stdout" not in captured.out
     assert "inside-stderr" not in captured.err
+
+
+def test_echo_via_pager_in_cli_runner() -> None:
+    """CliRunner should not get 'I/O operation on closed file' after echo_via_pager."""
+
+    @click.command()
+    def cli() -> None:
+        click.echo_via_pager("Hello, pager!")
+
+    runner = CliRunner()
+    result = runner.invoke(cli)
+    assert result.exit_code == 0
+    assert "Hello, pager!" in result.output