Drain the pty master in ScreenTests so endwin() does not hang on macOS

ScreenTests drives curses over a pseudo-terminal whose master was never
read.  On macOS (and other BSD-derived platforms) the tcdrain() that
curses performs inside endwin() blocks until the master side is read, so
the test hung until the timeout; Linux does not block there.

Drain each pty master in a background daemon thread, joined (with a
timeout) once both ends are closed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/test_curses.py

@@ -4,12 +4,13 @@
 import string
 import sys
 import tempfile
+import threading
 import unittest
 from unittest.mock import MagicMock
 
 from test.support import (requires, verbose, SaveSignals, cpython_only,
                           check_disallow_instantiation, MISSING_C_DOCSTRINGS,
-                          gc_collect)
+                          gc_collect, SHORT_TIMEOUT)
 from test.support.import_helper import import_module
 
 # Optionally test curses module.  This currently requires that the
@@ -1784,8 +1785,27 @@ def tearDown(self):
             pass
         gc_collect()
 
+    @staticmethod
+    def _drain_pty(master):
+        # Read and discard whatever curses writes to the screen.
+        try:
+            while os.read(master, 1024):
+                pass
+        except OSError:
+            pass
+
     def make_pty(self):
         master, slave = os.openpty()
+        # Drain the master in the background.  Otherwise writing to the slave
+        # -- and in particular the tcdrain() that curses performs inside
+        # endwin() -- blocks once the unread output fills the pty buffer on
+        # platforms such as macOS (Linux does not block here).
+        reader = threading.Thread(target=self._drain_pty, args=(master,),
+                                  daemon=True)
+        reader.start()
+        # Closing both ends makes the reader see EOF (or fail) and exit; join
+        # with a timeout so a stuck reader can never hang the test.
+        self.addCleanup(reader.join, SHORT_TIMEOUT)
         self.addCleanup(os.close, master)
         self.addCleanup(os.close, slave)
         return slave