Sovled: Users here suggested https://github.com/taj-ny/kwin-effects-forceblur which works wonderfully, even with video wallpapers!

Technically I’m sure it’s possible, I’ve seen programs that have a kind of frosted see-through look. But that’s something the program has to implement.

It’s possible at the application level for Qt stuff at least, iirc you just pass a window arg to enable transparency. If you mean from like a user settings perspective, idk sorry 😂

Could you grab the desktop background image, then do some trickery rendering the image using the window position to pan and crop it?