@burnout426 It shouldn't happen at all !! It's annoying and intrusive.
It can be stopped temporarily by renaming opera_gx_splash.exe but that's only until the next update - Opera's sloppy update process simply recreates a new version number folder with a new opera_gx_splash.exe but leaves the old version folders intact. Sloppy.
Currently looking for something else (besides Chrome, Ffox, Edge all of which are now jumping on the in-your-face trumpet fanfare bandwagon Lookit me, I've just updated myself