Current Scene abstractions served us well on X11, but they are not suitable on Wayland.
We have lots of subsurface-related problems:
- Cross-fading is broken;
- The whole subsurface tree will be repainted if any subsurface is damaged. This doesn't make sense if only a desync subsurface was damaged for example;
- Effects that transform window grid(e.g. Wobbly Windows) can't transform subsurfaces;
- Because sub-surfaces can't be clipped in some cases, we have bugs like https://bugs.kde.org/show_bug.cgi?id=387313.
So, in general, subsurface support is broken in KWin/Wayland.
The new Scene abstractions have to be suitable on both X11 and Wayland as well address previously mentioned issues.