Pablo (and Ned) 

PR_26672 (and backports) added one line to idlelib/autocomplete_w.py, 206 or 209.
  acw.update_idletasks() # Need for tk8.6.8 on macOS: #40128.
NEWs item included "The added update_idletask call should be harmless and possibly helpful otherwise. "

This is consistent with https://www.tcl.tk/man/tcl8.6/TclCmd/update.html

At the end of the PR, Tal Einat reports
"this broke completions for me with Tcl/Tk 8.6.11 on Ubuntu 20.04.
Apparently also with Tcl/Tk 8.6.10, which is the current default.
Also broken in my latest install of Python 3.9.7 (not built from source)."

Tal, did you verify that #ing the line fixes your issues?  Can you be more specific as to what you see?

I rechecked on my Macbook and the line is definitely needed for 3.9.7 with tk 8.6.8.  It makes no difference for 3.10.0rc2 with 8.6.11.  (So the bug of requiring update_idletasks call to ever see the dropdown box was fixed.)

Assume Tal's report is verified on another machine, we could, before 3.10.0,
1. Remove the line.
2. Disable the line.
3. Make it conditional on macOS and 8.6.8.

It is obviously too late for 3.9.7.