bpo-39885: IDLE context menu clears selection (GH-18859)

Since clicking to get an IDLE context menu moves the cursor,
any text selection should be and now is cleared.
(cherry picked from commit 4ca060d8ad)

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

Lib/idlelib/NEWS.txt

@@ -3,6 +3,9 @@ Released on 2019-12-16?
 ======================================
 
 
+bpo-39885: Since clicking to get an IDLE context menu moves the
+cursor, any text selection should be and now is cleared.
+
 bpo-39852: Edit "Go to line" now clears any selection, preventing
 accidental deletion.  It also updates Ln and Col on the status bar.
 

Lib/idlelib/editor.py

@@ -499,6 +499,7 @@ 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))
         if not self.rmenu:
             self.make_rmenu()

Misc/NEWS.d/next/IDLE/2020-03-08-14-27-36.bpo-39885.29ERiR.rst

@@ -0,0 +1,2 @@
+Since clicking to get an IDLE context menu moves the cursor,
+any text selection should be and now is cleared.