bpo-39885: Make IDLE context menu cut and copy work again (GH-18951)

Leave selection when right click within.  This exception to clearing selections when right-clicking was omitted from the previous commit, 4ca060d.  I did not realize that this completely disabled the context menu entries, and  I should have merged a minimal fix immediately.  An automated test should follow.
(cherry picked from commit 97e4e0f53d6690db6b942678489716a30925b8af)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

1 files changed, 11 insertions, 4 deletions

diff --git a/Lib/idlelib/editor.py b/Lib/idlelib/editor.py
index b0f88b5..a178eaf 100644
--- a/Lib/idlelib/editor.py
+++ b/Lib/idlelib/editor.py
@@ -499,15 +499,23 @@ class EditorWindow(object):
     rmenu = None
 
     def right_menu_event(self, event):
-        self.text.tag_remove("sel", "1.0", "end")
-        self.text.mark_set("insert", "@%d,%d" % (event.x, event.y))
+        text = self.text
+        newdex = text.index(f'@{event.x},{event.y}')
+        try:
+            in_selection = (text.compare('sel.first', '<=', newdex) and
+                           text.compare(newdex, '<=',  'sel.last'))
+        except TclError:
+            in_selection = False
+        if not in_selection:
+            text.tag_remove("sel", "1.0", "end")
+            text.mark_set("insert", newdex)
         if not self.rmenu:
             self.make_rmenu()
         rmenu = self.rmenu
         self.event = event
         iswin = sys.platform[:3] == 'win'
         if iswin:
-            self.text.config(cursor="arrow")
+            text.config(cursor="arrow")
 
         for item in self.rmenu_specs:
             try:
@@ -520,7 +528,6 @@ class EditorWindow(object):
             state = getattr(self, verify_state)()
             rmenu.entryconfigure(label, state=state)
 
-
         rmenu.tk_popup(event.x_root, event.y_root)
         if iswin:
             self.text.config(cursor="ibeam")