2 files changed, 5 insertions, 0 deletions
diff --git a/Lib/idlelib/PyShell.py b/Lib/idlelib/PyShell.py
index 3c203d0..106e32b 100755
--- a/Lib/idlelib/PyShell.py
+++ b/Lib/idlelib/PyShell.py
@@ -1384,6 +1384,9 @@ class PseudoInputFile(PseudoFile):
         line = self._line_buffer or self.shell.readline()
         if size < 0:
             size = len(line)
+        eol = line.find('\n', 0, size)
+        if eol >= 0:
+            size = eol + 1
         self._line_buffer = line[size:]
         return line[:size]
 
diff --git a/Misc/NEWS b/Misc/NEWS
index 8a7da09..bb9c6a9 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -240,6 +240,8 @@ Library
 IDLE
 ----
 
+- Issue #20058: sys.stdin.readline() in IDLE now always returns only one line.
+
 - Issue #19481: print() of string subclass instance in IDLE no longer hangs.
 
 - Issue #18270: Prevent possible IDLE AttributeError on OS X when no initial