I'm looking for a way to create a "Fullscreen window" KWin effect. The original idea and request was started here: https://bugs.kde.org/show_bug.cgi?id=510631

The basic task is to render a content of any window upscaled to fullscreen (activated by a keyboard shortcut like "Alt+Enter"). If possible, optionally applying a scaling shader (CRT, xBR etc.).

The motivation is to correctly display old games, which do not support native screen resolution, on a modern big displays. Unlike classic fullscreen mode, this would allow fine control of the output (aspect ratio, integer scaling, or ultimately even shaders) while it wouldn't need to actually switch the display mode (which is currently not possible on Wayland anyway). It would also enable a seamless task switching between the games and the desktop. Another possible usecase might be to scale an old video using the CRT shader etc.

I've briefly looked at the possibilities and it seems that this currently cannot be done in the QML effect, because of the need to handle the mouse when the window is scaled to fullscreen.
Creating such effect in C++ seems possible (by partialy re-using code from the magnifier effect), but I'm not sure, if such feature has a chance to be included in the core KWin effects.

Maintaining an out of tree C++ effect for KWin would be a pain, so the other possibility would be to add the needed functionality to QML effects. I can then create it as an external downloadable QML effect.
The missing features in QML effects are:
- Hide the system mouse cursor
- Draw a new one scaled to proper size and coords including the original shape (just like in the built-in magnifier)

More background and already considered solutions:
Lets say we want to play an original "Age of Empires" game on a modern Plasma desktop running on Wayland and a big ultrawide screen.
Currently the game works flawless in Wine, except it shows only a small picture in top-left corner of the screen. The max supported resolution in the game is 10124x768.

- There are wrappers like DxWrapper or dgVoodoo, which can hack the game itself and scale it to fullscreen, but not every game work great this way, you need to handle every app separately. Also, I've noticed issues when switching from the running game back to desktop.

- You can use the built-in screen magnifier with mouse tracking turned off, but you can't zoom exactly to match the window size and it is not very comfortable.

- What you can do with any of the games is to make them run in a windowed mode. If the game doesn't have a built-in window mode, you can use Wine desktop.

- Having the "Fullscreen effect" in KWin would allow for a great universal solution for any type of apps (native, Wine, DirectX, OpenGL, even DosBox, emulators etc.).