I could probably have written this in a clearer way. Child windows like a proto-compositor in a way, just with more limitations. You can put video/3D/etc content in a child window, but there are serious tradeoffs. When it's in a scrolling container, either your scrolling implementation is built from child windows, or you're going to deal with less than perfect synchronization as you overlay a changing clip/translate for the embedded content with draws of the rest of your UI (this used to be a common problem in browsers). Modern compositor APIs have explicit transactions[1, 2] causing the various view changes to be applied at the same time. It's not that the problem can't be solved, it's that it's trickier than people imagine.

And of course a Wayland subsurface is using the compositor.

[1]: https://docs.microsoft.com/en-us/windows/win32/directcomp/ba... [2]: https://developer.apple.com/documentation/quartzcore/catrans...