Each time I've installed it, it has been for all users, and was installed using Administrator privileges

Didn't you say that you've installed Opera using a non-admin account?

I think I have confused the terminology--sorry. I have always installed it for all users using administrator privileges to run the installer program. But Opera itself has always been run as a standard user account, except for when I was running it in administrator mode trying to eliminate the UAC popups. The popups occurred regardless of whether it was being run as administrator or as a standard user. In this final test it is being run as a standard user, but was installed in the unique (non-default) folder. Prior to about October 15 it was being run as a standard user, installed in the default folder and was not causing the UAC popups.
I hope this clarifies the situation.

Any idea why it happened that way?

I don't know for sure but I guess that Windows limits what non-administrators can install/use out of their user account directories.
Did you install Opera using the option 'for all users' or the 'for current user' one?

Each time I've installed it, it has been for all users, and was installed using Administrator privileges. Another thing to remember is that I have used Opera for quite some time. It was working perfectly normally until something started causing the UAC popups maybe around October 15. Prior to that I had never had any problems like that.

Now for a question: do you have any idea why installing in a different folder fixed the UAC popup issue?

Did you do it while using a non-administrator account?

I was using a non-administrator account when the original trouble first started. On this reinstallation in the different folder I was also using a non-administrator account. The odd thing is that when I first reinstalled it into the default folder I was also using a non-administrator account but the UAC popups were still happening. As I described, however, when I installed it again (into the unique folder) the popups disappeared. Any idea why it happened that way?

. . . could you do a regular installation but changing the installation folder to anything other than Program Files? It could be Opera Test or something like this.
Then run Opera from there and check if the problem still happens.

OK, I tried your suggestion and installed the program in a separate unique folder. On my first test, the program is working the way it should--no UAC popup is appearing. Before I did that, however, I uninstalled and reinstalled in the regular default program folder. In that case, the UAC popup did appear.
Now for a question: do you have any idea why installing in a different folder fixed the UAC popup issue?
Thanks for all your assistance.

The problem first started when it was not run as administrator. Switching to run as administrator was one thing that a web search suggested in order to try to stop the UAC popup. Thanks.

Launching the non-portable version of Opera still raises the UAC popup.

OK, so it seems to be an issue with Opera profile.
Aa a laat test, could you do a regular installation but changing the installation folder to anything other than Program Files? It could be Opera Test or something like this.
Then run Opera from there and check if the problem still happens.

Before I do this last suggested test, could you tell me why you think the problem is with the Opera profile since using the portable profile produced the same results when I used it in the regular desktop program? The portable program works fine when I launch it from its USB location. But when I copy that profile into the regular desktop installation, the desktop installation still triggers the UAC popup. I certainly don't mind doing the test, just trying to be sure I understand what I'm doing. Also, when I do this last test, should I uninstall my existing installation of Opera?

Ok, renamed the folder as you suggested and a new one was created. Launching the non-portable version of Opera still raises the UAC popup. And it also pops up when I click on a link in an email using Opera as the default browser. I appreciate all of your suggestions. Any other ideas on what to try? (I still have not tried a reinstallation).
I also tried taking the profile folder from the portable installation and using it in the desktop program by copying and renaming the portable profile into the desktop. Again, the UAC popup immediately came up when I launched the program.
Frustrating!!

Using the portable installation, the UAC popup does not appear when I launch Opera. I don't know how to make the portable version the default so that I can test links from emails. Since the regularly-installed version does cause the UAC popup to appear on launch at least this is progress. Perhaps I should do a complete uninstall and reinstallation of Opera?

Yes, regular installation with no changes--just a routine update from one version to the next. Yes, Opera is listed in the programs, and it is installed in C:\ProgramFiles(x86). The thing I find interesting is that I also use Opera on another Windows 10 computer and I am not experiencing this issue on it. The major difference is that the computer with the issue is running Windows 10 Pro, while the computer that does not exhibit the problem is running Windows 10 Home. This leads me to believe that it is some obscure problem related to that. I have compared all the settings I know to compare between the two computers and have found no differences.

Could you elaborate on your question? I downloaded the file from Opera and ran the setup file and let it do its thing. It was an update to the previous version.