If nothing’s changed, the Gtk 4 design tool Cambalache[1] by the former maintainer of the now-defunct Glade project uses[2] Broadway to render its design view, because it was there and an embedded Wayland compositor widget wasn’t. Broadway is awesome, but this just makes me sad.
That the easiest way for a UI design tool using a native toolkit and targeting that same toolkit to display a preview of the UI being edited turned out to be embedding the 800-pound gorilla called a webview.
The only reason the tool does that is to support multiple versions of GTK. But the alternative you suggest, embedding a Wayland compositor in a widget, doesn't exactly seem to be the lightest-weight of options.
Not my suggestion, for what it’s worth, but the author’s own (in ref. 2 above). And yes, it’s not precisely fantastic, but on the other hand “embedded Wayland compositor” is for the most part a fancy way to say “dumb RPC passthrough and a single blit”. (That doesn’t mean that it wouldn’t take a whole lot of work to make, just that the eventual result would not be huge.)
[1] https://gitlab.gnome.org/jpu/cambalache
[2] https://blogs.gnome.org/xjuan/2021/05/18/merengue-cambalache...