Flush the pager's stdin after each write

src/click/_termui_impl.py

@@ -466,6 +466,7 @@ def _pipepager(
                 text = strip_ansi(text)
 
             c.stdin.write(text)
+            c.stdin.flush()
     except BrokenPipeError:
         # In case the pager exited unexpectedly, ignore the broken pipe error.
         pass

tests/test_utils.py

@@ -1,8 +1,11 @@
+import io
 import os
 import pathlib
 import stat
 import subprocess
 import sys
+import threading
+import time
 from collections import namedtuple
 from contextlib import nullcontext
 from decimal import Decimal
@@ -418,6 +421,72 @@ def test_echo_via_pager(monkeypatch, capfd, pager_cmd, test):
     )
 
 
+def test_echo_via_pager_flushes_incremental_output(monkeypatch, tmp_path):
+    pager_output = tmp_path / "pager-output.txt"
+
+    saw_first_line_before_second_yield = False
+
+    class FakePagerProcess:
+        def __init__(self, output_path: Path) -> None:
+            self._output_path = output_path
+            read_fd, write_fd = os.pipe()
+            self.stdin = io.TextIOWrapper(
+                io.BufferedWriter(os.fdopen(write_fd, "wb", buffering=0)),
+                encoding="utf-8",
+            )
+            self._reader = io.TextIOWrapper(
+                os.fdopen(read_fd, "rb", buffering=0),
+                encoding="utf-8",
+            )
+            self._thread = threading.Thread(target=self._consume, daemon=True)
+            self._thread.start()
+
+        def _consume(self) -> None:
+            with self._reader, self._output_path.open("w", encoding="utf-8") as f:
+                while True:
+                    line = self._reader.readline()
+                    if line == "":
+                        break
+
+                    f.write(line)
+                    f.flush()
+
+        def terminate(self) -> None:
+            if not self.stdin.closed:
+                self.stdin.close()
+
+        def wait(self) -> int:
+            self._thread.join(timeout=1)
+            return 0
+
+    def fake_popen(*args, **kwargs):
+        return FakePagerProcess(pager_output)
+
+    def generator():
+        nonlocal saw_first_line_before_second_yield
+
+        yield "first line\n"
+
+        for _ in range(20):
+            if pager_output.exists() and pager_output.read_text() == "first line\n":
+                saw_first_line_before_second_yield = True
+                break
+
+            time.sleep(0.01)
+
+        yield "second line\n"
+
+    monkeypatch.setattr(subprocess, "Popen", fake_popen)
+
+    assert click._termui_impl._pipepager(
+        generator(),
+        ["cat"],
+        color=False,
+    )
+    assert saw_first_line_before_second_yield
+    assert pager_output.read_text() == "first line\nsecond line\n"
+
+
 def test_echo_color_flag(monkeypatch, capfd):
     isatty = True
     monkeypatch.setattr(click._compat, "isatty", lambda x: isatty)