Do not depend on the text geometry in RMenuTest

Text.bbox() returns None while the window is not mapped, as on Windows,
where the root window of the test is withdrawn.  Ask the widget for the
index of the clicked character instead of computing the coordinates of
a known index.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/idlelib/idle_test/test_editor.py

@@ -235,39 +235,47 @@ def tearDownClass(cls):
     class DummyRMenu:
         def tk_popup(x, y): pass
 
-    def click(self, index):
-        "Simulate a right click at the start of index; return the event."
-        x, y = self.text.bbox(index)[:2]
+    def click(self, x=0, y=0):
+        """Simulate a right click at the (x, y) pixel of the text.
+
+        Return the index of the clicked character, as computed by the
+        widget itself.  It cannot be computed here, because the geometry
+        of the text is unknown while its window is not mapped.
+        """
+        index = self.text.index(f'@{x},{y}')
         Event = namedtuple('Event', ['x', 'y', 'x_root', 'y_root'])
         event = Event(x, y, x_root=0, y_root=0)
         self.assertEqual(self.window.right_menu_event(event), 'break')
-        return event
+        return index
 
     def test_rclick_no_selection(self):
         text = self.text
         insert(text, 'one two three')
-        self.click('1.8')
+        index = self.click()
         self.assertEqual(text.tag_ranges('sel'), ())
-        self.assertEqual(text.index('insert'), '1.8')
+        self.assertEqual(text.index('insert'), index)
 
     def test_rclick_outside_selection(self):
         text = self.text
         insert(text, 'one two three')
-        text.tag_add('sel', '1.0', '1.3')
-        text.mark_set('insert', '1.3')
-        self.click('1.8')
+        # The selection does not contain the clicked character.
+        text.tag_add('sel', '1.5', '1.8')
+        text.mark_set('insert', '1.8')
+        index = self.click()
         self.assertEqual(text.tag_ranges('sel'), ())
-        self.assertEqual(text.index('insert'), '1.8')
+        self.assertEqual(text.index('insert'), index)
 
     def test_rclick_inside_selection(self):
         text = self.text
         insert(text, 'one two three')
-        text.tag_add('sel', '1.4', '1.7')
-        text.mark_set('insert', '1.7')
-        self.click('1.5')
-        self.assertEqual(text.index('sel.first'), '1.4')
-        self.assertEqual(text.index('sel.last'), '1.7')
-        self.assertEqual(text.index('insert'), '1.7')
+        # The selection contains the clicked character.
+        index = text.index('@0,0')
+        text.tag_add('sel', index, f'{index}+3c')
+        text.mark_set('insert', 'end-1c')
+        self.click()
+        self.assertEqual(text.index('sel.first'), index)
+        self.assertEqual(text.index('sel.last'), text.index(f'{index}+3c'))
+        self.assertEqual(text.index('insert'), text.index('end-1c'))
 
     def test_rmenu_check_copy(self):
         text = self.text