Bingo! I think you have nailed it. This extension was installed:
https://addons.opera.com/en/extensions/details/google-calendar/

I do not think it ever functioned properly, as I never noticed anything in the browser that related to it. I disabled it, and the black screen has not appeared for 2 hours or so.

I guess one question that still exists is why it would cause the black screen effect?

Thanks, and I'll confirm after more testing.

the screenshot is at: https://postimg.org/image/7f1305oz1/

In addition to the link in the title bar, it also changes to read Sign in - Google accounts

I can't see any direct connection. It occurs when only this page is open in the browser.

I have managed to save a screenshot of the issue, but unsure how to share this.

It is trying to connect with Google Accounts (I am not signed in, although it happens when signed in as well)

Running Malwarebytes, Spybot, and Windows Defender

Updated, and still an issue. Seems like it happens when connecting to a Google site in the background

Hardware acceleration is disabled

Version 40.0.2308.81

It is just the pane.

It is unclear if it would be clickable. I haven't tried as the duration is short, and no snapshot for the same reason.

Periodically, my Opera browser just changes to a black screen, although this usually lasts less than five seconds. It says Google within the top title bar, so is it checking something there? This happens whether signed into a Google account or not.

Any suggestions for a fix?