Oh, ok, so X apps can run under Wayland, because Wayland DEs can run an X server (an existing X impl, or does Wayland provide an X translation layer/impl?)
But the DE/window manager is what is actually responsible for deciding when/where to provide that interface? For actual rendering+windows etc the DE is communicating entirely through Wayland specific APIs?
I guess the operation mode is similar to how you would run X apps on Mac and windows where you could launch an x server that would act as a layer between the Mac/windows windowing system?
Wayland is written by most of the same folks who maintain xorg, and XWayland is part of xorg.
It's generally more transparent than running a separate X server on Mac or Windows -- you don't normally need to care about whether an app is X11 or native Wayland.
Xwayland is basically Xorg with a Wayland DDX layer -- and it recently got a rootful mode. So that means it's possible to run a complete X desktop, including WM, on top of a Wayland compositor.
However, the drawback is that with the exception maybe of Weston, one does not simply start up a Wayland compositor. You need to have support for logind seats and that sort of thing. So unless you want to spend an afternoon assembling the components like a piece of flat-pack furniture from Target, you basically need systemd and a distro that bundled those components for you the right way.
Not really. It's perfectly fine to just advertise a single dummy wl_seat, and use whatever non-logind mechanism there is to tell the OS that you want to be in control of the input and display hardware.
You can run any wlroots- (and possibly smithay-) based Wayland compositor nested inside an X11, but it won't be rootless; you'll get a separate window that all the Wayland apps will be drawn into.
There's really no reason why someone couldn't write a "WX11" or something of that nature, but I'm not sure it would be worth the effort.
But the DE/window manager is what is actually responsible for deciding when/where to provide that interface? For actual rendering+windows etc the DE is communicating entirely through Wayland specific APIs?
I guess the operation mode is similar to how you would run X apps on Mac and windows where you could launch an x server that would act as a layer between the Mac/windows windowing system?