bpo-40128: Fix IDLE autocomplete on macOS (GH-26672)

In particular, when running with tk8.6.8, as in PSF 3.9.

Co-authored-by: Terry Jan Reedy <[email protected]>
(cherry picked from commit 3ec3ee7)

Co-authored-by: Kaustubh J <[email protected]>

Author: miss-islington

Lib/idlelib/autocomplete_w.py

@@ -206,6 +206,7 @@ def show_window(self, comp_lists, index, complete, mode, userWantsWin):
         scrollbar.config(command=listbox.yview)
         scrollbar.pack(side=RIGHT, fill=Y)
         listbox.pack(side=LEFT, fill=BOTH, expand=True)
+        acw.update_idletasks() # Need for tk8.6.8 on macOS: #40128.
         acw.lift()  # work around bug in Tk 8.5.18+ (issue #24570)
 
         # Initialize the listbox selection

Misc/NEWS.d/next/IDLE/2021-06-11-17-43-39.bpo-40128.7vDN3U.rst

@@ -0,0 +1,3 @@
+Mostly fix completions on macOS when not using tcl/tk 8.6.11 (as with 3.9).
+The added update_idletask call should be harmless and possibly helpful
+otherwise.