diff --git a/eden/scm/sapling/ext/interactiveui.py b/eden/scm/sapling/ext/interactiveui.py
index 31f3a6ee85d51..c193beef5a500 100644
--- a/eden/scm/sapling/ext/interactiveui.py
+++ b/eden/scm/sapling/ext/interactiveui.py
@@ -10,7 +10,7 @@
 import os
 import sys
 from enum import Enum
-from typing import Union
+from typing import Callable, Union
 
 from sapling import error, scmutil, util
 from sapling.i18n import _, _x
@@ -73,26 +73,34 @@ def clearscreen(out):
 # Note: some changes have been made from the source code
 
 
-def getchar() -> Union[None, bytes, str]:
-    fd = sys.stdin.fileno()
+def _readraw(fd: int) -> bytes:
+    """read the bytes currently available on `fd` with the tty in raw mode"""
+    attr = termios.tcgetattr(fd)
+    try:
+        tty.setraw(fd)
+        return os.read(fd, 32)
+    finally:
+        termios.tcsetattr(fd, termios.TCSADRAIN, attr)
+
+
+def getchar(
+    stdin=None, readraw: Callable[[int], bytes] = _readraw
+) -> Union[None, bytes, str]:
+    # `stdin` and `readraw` are seams for tests. Production callers leave them
+    # unset and get sys.stdin plus the real raw terminal read.
+    if stdin is None:
+        stdin = sys.stdin
+    fd = stdin.fileno()
     if not os.isatty(fd):
-        # TODO: figure out tests
         return None
+    ch = None
     try:
-        attr = termios.tcgetattr(fd)
-        try:
-            tty.setraw(fd)
-            ch = os.read(fd, 32)
-        finally:
-            termios.tcsetattr(fd, termios.TCSADRAIN, attr)
+        ch = readraw(fd)
     except termios.error:
-        # pyre-fixme[61]: `ch` is undefined, or not always defined.
         if ch is None:
             ch = ""
-    # pyre-fixme[61]: `ch` is undefined, or not always defined.
     if ch == b"\x03" or ch == b"\x04":
         return None
-    # pyre-fixme[61]: `ch` is undefined, or not always defined.
     return ch
 
 
diff --git a/eden/scm/tests/test-interactiveui.py b/eden/scm/tests/test-interactiveui.py
new file mode 100644
index 0000000000000..c06ace058a278
--- /dev/null
+++ b/eden/scm/tests/test-interactiveui.py
@@ -0,0 +1,102 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2.
+
+import os
+import unittest
+
+import silenttestrunner
+from sapling import util
+from sapling.ext import interactiveui
+
+
+class fakestdin:
+    """stands in for sys.stdin, backed by a real fd"""
+
+    def __init__(self, fd):
+        self._fd = fd
+
+    def fileno(self):
+        return self._fd
+
+
+@unittest.skipIf(util.iswindows, "interactiveui does not support Windows")
+class testgetchar(unittest.TestCase):
+    def setUp(self):
+        # a pipe is a real non-tty fd; a pty secondary is a real tty fd
+        self.pipefd, writefd = os.pipe()
+        self.addCleanup(os.close, self.pipefd)
+        self.addCleanup(os.close, writefd)
+        primary, self.ttyfd = os.openpty()
+        self.addCleanup(os.close, self.ttyfd)
+        self.addCleanup(os.close, primary)
+
+    def _getchar(self, keys, fd=None):
+        """run getchar() against `fd`, with the raw read faked to yield `keys`
+
+        Returns the getchar() result and the fds the read was attempted on, so
+        tests can assert the terminal is left alone on the non-tty path.
+        """
+        reads = []
+
+        def readraw(fd):
+            reads.append(fd)
+            return keys
+
+        result = interactiveui.getchar(
+            stdin=fakestdin(self.ttyfd if fd is None else fd), readraw=readraw
+        )
+        return result, reads
+
+    def testnottty(self):
+        result, reads = self._getchar(b"j", fd=self.pipefd)
+        self.assertIsNone(result)
+        # the terminal must not be touched at all when stdin is not a tty
+        self.assertEqual(reads, [])
+
+    def testkeypress(self):
+        result, reads = self._getchar(b"j")
+        self.assertEqual(result, b"j")
+        self.assertEqual(reads, [self.ttyfd])
+
+    def testinterrupt(self):
+        # ctrl-c and ctrl-d end the session rather than returning a keypress
+        self.assertIsNone(self._getchar(b"\x03")[0])
+        self.assertIsNone(self._getchar(b"\x04")[0])
+
+    def testescapesequence(self):
+        result, _reads = self._getchar(b"\x1b[A")
+        self.assertEqual(result, interactiveui.viewframe.KEY_UP)
+        self.assertEqual(
+            interactiveui._splitkeypresses(result),
+            [interactiveui.viewframe.KEY_UP],
+        )
+
+    def testescapesequencerun(self):
+        # a single read can deliver several arrow keys plus a normal key
+        result, _reads = self._getchar(b"\x1b[A\x1b[Dj")
+        self.assertEqual(
+            interactiveui._splitkeypresses(result),
+            [
+                interactiveui.viewframe.KEY_UP,
+                interactiveui.viewframe.KEY_LEFT,
+                b"j",
+            ],
+        )
+
+
+class testrealread(unittest.TestCase):
+    def testnottty(self):
+        # the default readraw is never reached for a non-tty, so this exercises
+        # the real production code path end to end
+        readfd, writefd = os.pipe()
+        try:
+            self.assertIsNone(interactiveui.getchar(stdin=fakestdin(readfd)))
+        finally:
+            os.close(readfd)
+            os.close(writefd)
+
+
+if __name__ == "__main__":
+    silenttestrunner.main(__name__)