Another awesome aspect of X11: Xembed. Since it's "windows all the way down" you can have apps (rather than the window manager or root window) be the parent of other app windows.
Two nifty examples of this:
Two nifty examples of this:
For a more practical way to simulate a top-level window being a child window of another program, you make the top-level window Borderless, and perform all the necessary window management to maintain position, dimensions, and Z-order.
Anyway, I believe the Microsoft Management Console (MMC) actually does this. The content windows (at least in some cases) belong to a different process than the surrounding UI.
Also, WebView2 (the new Chromium-based Edge webview) runs out-of-process, but maybe it does some things in the embedding process to improve performance?
As for why there would be bad performance for putting a window into another process's window, I can only speculate. It takes something like 10000+ CPU cycles for Windows to finish switching to a different thread or process. If this is happening for every message that is handled by both the parent and the child window, that would be a lot of overhead.
I haven't checked if the Out-of-process webview is the EXE that owns the window or not.
No idea how that's gonna work on Wayland.