The completion window never appears with Python version 3.9.7 and with the current main branch.

Ubuntu 20.04 (reproduced on two separate machines)
Tested with Tcl/Tk versions 8.6.10 and 8.6.11.

This is directly caused by the fix for issue #40128. Commenting out that line resolves this issue entirely.

(See also the PR for that fix, PR GH-26672.)

I've also tested this on Windows 10 with the latest main branch (to be 3.11). The completions window works as expected with or without the recently added .update_idletasks() call.

I do recommend limiting this to macOS (platform.system() == "Darwin"). 

Ideally we'd test this on macOS with several versions of tcl/tk (8.6.8, 8.6.9, 8.6.10, 8.6.11) and limit it to only those versions where the fix is required:

TK_VERSION = tuple(map(int, tk.call("info", "patchlevel").split(".")))
if (8, 6, 8) <= TK_VERSION < (8, 6, 10):
    acw.update_idletasks()

Also we might check if calling .update() is enough, since that does less than .update_idletasks().

See PR GH-28328 with proposed fix. It should be tested on macOS with several relevant versions of Tcl/Tk to ensure that the Tk version range used is correct.

In msg401744 I suggested any of the following for the upcoming 3.10.0 release.

1. Remove the new line.
2. Disable the line by adding '#'.
3. Make it conditional on macOS and 8.6.8.

Thinking more, I am rejecting 3. because the line is not needed for IDLE and tk as installed by our macOS.  As things stand now, I an only willing to support IDLE with the tk we install.  But I will go with option 2 so that if someone tries to run IDLE with a different tk, and the line is needed, they only need to remove the '#'.

A version of 3. is needed for 3.9 but this needs more data and discussion.  In looking at your no-issue PR_28332, I realized that I would rather make a more extensive change than to patch a badly written function for at least the fourth time.  I will open an issue for that PR and explain later.

I will just mention here that winconfig_event also calls update_idletasks, in what should be the proper place, and I think that the second call was only needed for mac tk 8.6.8 because of the unique-to-IDLE behavior I want to delete.  I suspect that if we create the popup the same way we do with the others, we will not need the second call even on 8.6.8.  But making sure 3.10.0 works comes first.

New changeset 1afc7b3219b24c951bb4e6b7e1ead904228de074 by Terry Jan Reedy in branch 'main':
bpo-45193: Restore IDLE completion boxes on Ubuntu (GH-28343)
https://github.com/python/cpython/commit/1afc7b3219b24c951bb4e6b7e1ead904228de074

New changeset 0c64569ac7066a97e4482c6d6e4d780806692ae5 by Miss Islington (bot) in branch '3.10':
bpo-45193: Restore IDLE completion boxes on Ubuntu (GH-28343)
https://github.com/python/cpython/commit/0c64569ac7066a97e4482c6d6e4d780806692ae5

New changeset 9d76d28867c28bcc881b851547a9cd7ac003ae88 by Terry Jan Reedy in branch 'main':
bpo-45193: News for IDLE PR_28343 (GH-28348)
https://github.com/python/cpython/commit/9d76d28867c28bcc881b851547a9cd7ac003ae88

New changeset a5bc0ffc520e09226f85d5fa8faaa83be0acee68 by Miss Islington (bot) in branch '3.10':
bpo-45193: News for IDLE PR_28343 (GH-28348)
https://github.com/python/cpython/commit/a5bc0ffc520e09226f85d5fa8faaa83be0acee68

0c64569ac7066a97e4482c6d6e4d780806692ae5
a5bc0ffc520e09226f85d5fa8faaa83be0acee68

are ready to be cherrypicked into into the 3.10.0 release branch.  
Once that is done, 'release blocker' can be removed, but issue should remain open for a separate fix for 3.9.

> are ready to be cherrypicked into into the 3.10.0 release branch.  
Once that is done, 'release blocker' can be removed, but issue should remain open for a separate fix for 3.9.

Done! I am removing the release blocker and the 3.10, 3.11 version tags.

Thanks Tal and Terry for all the work!

New changeset 7f2d049cce6dc749a589424d4fb62ffca0b85b32 by Pablo Galindo (Miss Islington (bot)) in branch '3.10':
bpo-45193: Restore IDLE completion boxes on Ubuntu (GH-28343)
https://github.com/python/cpython/commit/7f2d049cce6dc749a589424d4fb62ffca0b85b32

New changeset d08e4a8c55ae3bd06c2e871265911ee88429b6bb by Pablo Galindo (Miss Islington (bot)) in branch '3.10':
bpo-45193: News for IDLE PR_28343 (GH-28348)
https://github.com/python/cpython/commit/d08e4a8c55ae3bd06c2e871265911ee88429b6bb