Clicking a mailto: link within Safari (current) switches to a running Opera to handle the mailto:$wudever. Investigation turns up that Mail.app/Settings/General/Default-mail-reader says Opera.app. I'm not aware that I approved that change. Fix by returning the setting to Mail.app or something else.

I also note that someone (not me) altered opera:/settings/handlers to point to mail.google.com <sigh> though I hesitate to assign responsibility.

I've been using Opera's computer browser since it was a paid app. Macos / Windows 10+ (and other virtual hosts) and every meaningful browser is present in my work environment; I used the mail client. I have long valued Opera's solid standards complaince and cross-platform support. I can't remember the last time I encountered a similarly disruptive feature in Opera.

HTH.