fix I/O operation on closed file error with CliRunner and echo_via_pager

CHANGES.rst

@@ -8,6 +8,7 @@ Unreleased
 -   Zsh completion scripts parse correctly on Windows. :issue:`3277`
 -   Shell completion of `Choice` `Enum` values produces a valid completion
     result. :issue:`3015`
+-   Fix I/O operation on closed file error with `echo_via_pager`. :issue:`3449`
 
 
 Version 8.4.0

src/click/_termui_impl.py

@@ -385,6 +385,24 @@ def generator(self) -> cabc.Iterator[V]:
             self.render_progress()
 
 
+class _MaybeStripAnsiThin:
+    def __init__(self, stream: t.TextIO, color: bool):
+        self.color = color
+        self.stream = stream
+
+    def write(self, text: str) -> int:
+        if not self.color:
+            text = strip_ansi(text)
+        return self.stream.write(text)
+
+    def flush(self) -> None:
+        return self.stream.flush()
+
+    def __getattr__(self, name: str) -> t.Any:
+        # Forward other attributes (e.g., fileno, isatty)
+        return getattr(self.stream, name)
+
+
 class MaybeStripAnsi(io.TextIOWrapper):
     def __init__(self, stream: t.IO[bytes], *, color: bool, **kwargs: t.Any):
         super().__init__(stream, **kwargs)
@@ -436,10 +454,14 @@ def get_pager_file(color: bool | None = None) -> t.Generator[t.TextIO, None, Non
                   default is autodetection.
     """
     with _pager_contextmanager(color=color) as (stream, encoding, color):
-        # Split streams by capabilities rather than the abstract TextIO /
-        # BinaryIO annotations: buffered text streams can be unwrapped to bytes,
-        # while text-only streams are yielded as-is.
-        if _has_binary_buffer(stream):
+        # route stdout and stderr through a thinner wrapper
+        is_std = stream is sys.stdout or stream is sys.stderr
+        if not isinstance(stream, t.BinaryIO) and is_std:
+            stream = _MaybeStripAnsiThin(stream, color=color)
+        # otherwise, split streams by capabilities rather than the abstract
+        # TextIO / BinaryIO annotations: buffered text streams can be unwrapped
+        # to bytes, while text-only streams are yielded as-is.
+        elif _has_binary_buffer(stream):
             # Text stream backed by a binary buffer.
             stream = MaybeStripAnsi(stream.buffer, color=color, encoding=encoding)
         elif isinstance(stream, t.BinaryIO):

tests/test_testing.py

@@ -256,6 +256,17 @@ def cli():
     assert result.output == ""
 
 
+def test_with_echo_via_pager():
+    @click.command()
+    def cli():
+        click.echo_via_pager("Hello, Click!")
+
+    runner = CliRunner()
+    result = runner.invoke(cli)
+    assert not result.exception
+    assert result.output == "Hello, Click!\n"
+
+
 def test_exit_code_and_output_from_sys_exit():
     # See issue #362
     @click.command()