diff -r cbb5c5da28f2 Lib/idlelib/idle_test/test_edit.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Lib/idlelib/idle_test/test_edit.py	Wed Jun 08 01:47:28 2016 -0400
@@ -0,0 +1,48 @@
+'''Test (selected) IDLE Edit menu items.
+
+Edit modules have their own test files files 
+'''
+from test.support import requires
+requires('gui')
+import tkinter as tk
+import unittest
+from idlelib import editor
+from idlelib import macosx
+from idlelib import pyshell
+
+class PasteTest(unittest.TestCase):
+
+    @classmethod
+    def setUpClass(cls):
+        cls.root = tk.Tk()
+        macosx._initializeTkVariantTests(cls.root)
+        pyshell.fix_x11_paste(cls.root)
+        cls.text = tk.Text(cls.root)
+
+    @classmethod
+    def tearDownClass(cls):
+        del cls.text
+        cls.root.destroy()
+        del cls.root
+
+    def tearDown(self):
+        self.text.delete('1.0', 'end')
+        self.text.clipboard_clear()
+        
+    def test_paste_noselection(self):
+        text = self.text
+        text.insert('insert', 'one')
+        text.clipboard_append('two')
+        text.event_generate('<<Paste>>')
+        self.assertEqual(text.get('1.0', 'end'), 'onetwo\n')
+
+    def test_paste_selection(self):
+        text = self.text
+        text.insert('insert', 'one', 'sel')
+        text.clipboard_append('two')
+        text.event_generate('<<Paste>>')
+        self.assertEqual(text.get('1.0', 'end'), 'two\n')
+
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2)
diff -r cbb5c5da28f2 Lib/idlelib/pyshell.py
--- a/Lib/idlelib/pyshell.py	Tue Jun 07 15:35:37 2016 -0400
+++ b/Lib/idlelib/pyshell.py	Wed Jun 08 01:47:28 2016 -0400
@@ -1396,6 +1396,17 @@
         self.shell.close()
 
 
+def fix_x11_paste(root):
+    "Make paste replace selection on x11.  See issue #5124."
+    if root._windowingsystem == 'x11':
+        for cls in 'Text', 'Entry', 'Spinbox':
+            root.bind_class(
+                cls,
+                '<<Paste>>',
+                'catch {%W delete sel.first sel.last}\n' +
+                        root.bind_class(cls, '<<Paste>>'))
+
+
 usage_msg = """\
 
 USAGE: idle  [-deins] [-t title] [file]*
@@ -1528,8 +1539,10 @@
                                     'editor-on-startup', type='bool')
     enable_edit = enable_edit or edit_start
     enable_shell = enable_shell or not enable_edit
+
     # start editor and/or shell windows:
     root = Tk(className="Idle")
+    root.withdraw()
 
     # set application icon
     icondir = os.path.join(os.path.dirname(__file__), 'Icons')
@@ -1544,7 +1557,7 @@
         root.wm_iconphoto(True, *icons)
 
     fixwordbreaks(root)
-    root.withdraw()
+    fix_x11_paste(root)
     flist = PyShellFileList(root)
     macosx.setupApp(root, flist)