Some of you might remember a couple weeks ago, when I complained that my Brave browser was displaying everything about 4 times normal size, and thus screen images were showing only a small part of what they should. That's the main reason I stopped using Brave on our media machine (Plasma).
After today's upgrades, Opera did the same thing. After some experimentation with the Display settings, I found out that at 1.50x, that no longer occurred. (I'd been using 2.0x because our TV is a 4K model.) Setting the Global Scale to 1.50x was a good workaround, but it left the screen contents a little smaller than I needed for comfortable reading from across the living room, so I tried setting it to 1.75x.
That turned out to be the ideal setting. Finally, I set it to 2.0x in order to see whether using the lower setting might have effected a change that would still apply at the higher setting, but once again everything was way too big, so I changed back to 1.75x and left it there.
I mention this in case anyone else here is using Plasma with a 4K screen, and has noticed the same issues. Changing the Global Scale setting to a slightly smaller value seems to be a really good workaround. But why it would affect some browsers and not everything else is just one of those things that makes me go, "Hmmm."