I can not replicate this

Neither do I. J4 nightly.

I can't replicate that either. I have created a group with two subgroups. I was able to delete them. Then I deleted the group editor because you gave this as an example. That was OK too.
Maybe you can narrow the error down?

Strange, I had it permanently with several fresh installations. Can you re-try deleting several groups (with child groups) at the same time? I just deletd one group and it was comme il faut. Then I deleted two groups at the same time and the issue was as described.

I tested this and everything, including the toolbar and the sidebar, was OK.

You're still experiencing the problem. What group names do you use? Maybe there is something irrelevant.

Can't reproduce but I didn't use a fresh installation.

but I think I found another issue????

1. Deleted 1 group "Publisher". OK. Message OK.

2. Then I tried to delete several groups in one go and checked them all. See image:

No message(!) and several groups left:

After several new installations, I cannot reproduce the issue. Also deleting several groups works fine now.
@ReLater I could not reproduce your issue. PCould you open a new issue for that?
Then I colud close this one.

Ah, close please. Not that important for me.

Set to "closed" on behalf of @jwaisner by The JTracker Application at issues.joomla.org/joomla-cms/28522

Closing as this is not reproducible.

Set to "open" on behalf of @jwaisner by The JTracker Application at issues.joomla.org/joomla-cms/28522

Confirmed on latest build. System info below:

PHP - 7.4.4
Database - MSQL 5.7.26
OS - Windows 10
Error Reporting - Maximum (no errors shown)

This was done with a complete wipe of all local data and SQL database.

My System Information for the tests last week

php: Linux astrid-TravelMate-5760G 4.15.0-96-generic #97-Ubuntu SMP Wed Apr 1 03:25:46 UTC 2020 x86_64
dbserver: mysql
dbversion: 5.7.29-0ubuntu0.18.04.1
dbcollation: utf8mb4_unicode_ci
dbconnectioncollation: utf8mb4_general_ci
dbconnectionencryption:
dbconnencryptsupported: true
phpversion: 7.2.29-1+ubuntu18.04.1+deb.sury.org+1
server: Apache/2.4.29 (Ubuntu)

I just could reproduce the issue by deleting the "Manager" user group (which deleted the administrator group too, because it's a child) on a fresh installed 4.0-dev without any sample data.

I don't know if it is related, but I noticed that the lftand rgtvalues in the user group table in database are not adjusted then the user group is deleted, see following 2 screenshots before and after.

Before:

After:

Patching the lftand rgtvalues in the database to the correct values doesn't help, the navigation menu, toolbar and so on is still disappeared.

Patched:

@ReLater Your finding above #28522 (comment) could be caused by what I found in my previous comment.

Btw. I saw nothing in PHP error log or database server log.

The issue I've found and mentioned in my comment above about lft and rgt values not being recalculated in the usergroups table is not related to this issue here and vice versa. My issue already exists in staging and 3.9.16. PR for my issue is #28684 .

Set to "closed" on behalf of @SharkyKZ by The JTracker Application at issues.joomla.org/joomla-cms/28522

Please test PR #28687.