When watching full screen videos or doing a full-screen task on a laptop, I often find that I need to check the clock and battery. To do so, currently, I have to exit the full-screen, and place the mouse over the battery icon in the system tray. I think it would be very convenient if there is an effect that shows the clock/battery at the top corer of the screen (like Japanese morning TV programmes), and I could toggle it on/off with a shortcut. I don't think it's too hard to implement, so I tried to create an effect myself, but I could not find enough resources to get started, so I could not even make a "hello world" effect. I am continue to search for ways, but in the meanwhile, if someone who knows effect programming well could do it, that would be better.
You can start from this: https://github.com/jinliu/kwin-effect-overlayclock documents: https://develop.kde.org/docs/plasma/kwin/
Looks like an interesting 3rd-party plugin opportunity! The use case seems fairly niche though, so I don't see this being added to KWin officially. Do feel free to work on it yourself though!