Worse is issue #2: There are only two options... close (the x on the left), and allow (the button on the right). There is no way to say "no, I don't want this." Meaning that, unless you click "allow," this alert will come up again every single time you go the calendar.

Just in case anyone else runs across this because they're having the same problem (like I just was), there is a way to fix this for a particular site yourself but it does require a fair bit of technical knowledge. If you don't know what a condensed JSON file is and how to edit one safely, you should just disable the option altogether as mentioned above.

If you do know what a condensed JSON file is and how to edit one, here's the rough outline. I'm intentionally leaving out a lot of details just in case someone is tempted to try this without knowing what they're doing and potentially corrupting their entire profile in the process.

1. Go to the site that's asking if you want to "Allow" it to be a protocol handler and click "Allow". I know this seems backwards but bear with me.
2. Completely close Opera. Otherwise the changes you're about to make will get lost.
3. Open up the "Preferences" file in your Opera profile directory. (preferably in an editor than can expand it to make it more readable. I used Atom with the pretty-json package.)
4. Search for "registered_protocol_handlers" and look for the one you just registered.
5. You should see the handler you just selected listed in that array. It should have 4 properties: default, last_modified, protocol, and url. Delete "default" (including the value and the trailing comma)
6. If you want to be extra thorough, move the entire block for that particular handler from the registered_protocol_handlers array into the ignored_protocol_handlers one. If you don't know what that means just skip this step.
7. Save the file and start Opera back up. Navigate to the site in question and you should find that it no longer prompts you. Note that when you accepted the handler earlier, you may have updated your system handler preferences as well (for mailto links for example). Remember to change that back after you're done, otherwise those links will still call Opera but it won't know what to do with them.

your answer seems like a non-sequitur, I think maybe you missed the distinction I was making between this forum and the "suggestion box forum", whatever/wherever that is... i.e. I was being told I should post somewhere else rather than here if I expect someone from Opera to see it.

The Suggestion box is here but it is another SubForum in General Topics.
Opera employees read Suggestion box posts but do not reply.
They mostly reply in Mac's subforum. :lol:

I though you knew about subforums but didn't read entirely the Sgunhouse's post.

Tufazay, thanks for that link, I was unfamiliar with that as well.

re: "Post about it in the Suggestion box forum to get someone's attention (though they may not reply, they will read it)."
They don't read the stuff in this forum? That would kind of defeat the purpose of reporting bugs here!

They read it, but do not reply in Suggestion box.

They don't read the stuff in this forum? That would kind of defeat the purpose of reporting bugs here!

Settings-->Privacy & Security-->Handlers

and selecting

"Do not allow any site to handle protocols"

This took me a while to discover, because I did not know what a "handler" was.

There does not appear to be a way to turn it off for one thing without turning it off for everything, though. (Nor does it explain the "information" icon that doesn't appear to do anything.)