bpo-37902: IDLE: Add scrolling for IDLE browsers. (#15368)

Modify the wheel event handler so it can also be used for module, path, and stack browsers.
Patch by George Zhang.

Lib/idlelib/idle_test/test_tree.py

@@ -4,7 +4,7 @@ from idlelib import tree
 import unittest
 from test.support import requires
 requires('gui')
-from tkinter import Tk
+from tkinter import Tk, EventType, SCROLL
 
 
 class TreeTest(unittest.TestCase):
@@ -29,5 +29,32 @@ class TreeTest(unittest.TestCase):
         node.expand()
 
 
+class TestScrollEvent(unittest.TestCase):
+
+    def test_wheel_event(self):
+        # Fake widget class containing `yview` only.
+        class _Widget:
+            def __init__(widget, *expected):
+                widget.expected = expected
+            def yview(widget, *args):
+                self.assertTupleEqual(widget.expected, args)
+        # Fake event class
+        class _Event:
+            pass
+        #        (type, delta, num, amount)
+        tests = ((EventType.MouseWheel, 120, -1, -5),
+                 (EventType.MouseWheel, -120, -1, 5),
+                 (EventType.ButtonPress, -1, 4, -5),
+                 (EventType.ButtonPress, -1, 5, 5))
+
+        event = _Event()
+        for ty, delta, num, amount in tests:
+            event.type = ty
+            event.delta = delta
+            event.num = num
+            res = tree.wheel_event(event, _Widget(SCROLL, amount, "units"))
+            self.assertEqual(res, "break")
+
+
 if __name__ == '__main__':
     unittest.main(verbosity=2)