Issue *24750: Switch all scrollbars in IDLE to ttk versions.

Where needed, add minimal tests to cover changes.

Lib/idlelib/idle_test/test_tree.py

@@ -0,0 +1,36 @@
+''' Test idlelib.tree.
+
+Coverage: 56%
+'''
+from idlelib import tree
+from test.support import requires
+requires('gui')
+import os
+import unittest
+from tkinter import Tk
+
+
+class TreeTest(unittest.TestCase):
+
+    @classmethod
+    def setUpClass(cls):
+        cls.root = Tk()
+        cls.root.withdraw()
+
+    @classmethod
+    def tearDownClass(cls):
+        cls.root.destroy()
+        del cls.root
+
+    def test_init(self):
+        # Start with code slightly adapted from htest.
+        sc = tree.ScrolledCanvas(
+            self.root, bg="white", highlightthickness=0, takefocus=1)
+        sc.frame.pack(expand=1, fill="both", side='left')
+        item = tree.FileTreeItem(tree.ICONDIR)
+        node = tree.TreeNode(sc.canvas, None, item)
+        node.expand()
+
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2)