diff --git a/Lib/idlelib/autocomplete_w.py b/Lib/idlelib/autocomplete_w.py
index dd20d9f8ec7..21b8a2472dd 100644
--- a/Lib/idlelib/autocomplete_w.py
+++ b/Lib/idlelib/autocomplete_w.py
@@ -203,6 +203,7 @@ 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
diff --git a/Misc/NEWS.d/next/IDLE/2021-06-11-17-43-39.bpo-40128.7vDN3U.rst b/Misc/NEWS.d/next/IDLE/2021-06-11-17-43-39.bpo-40128.7vDN3U.rst
new file mode 100644
index 00000000000..dafbe2cd5c3
--- /dev/null
+++ b/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.