For purposes like embedding a clock or timer window picture-in-picture while screensharing a presentation, KClock is adding Wayland picture-in-picture support for this functionality that will be available under KDE Plasma with the Wayland session but not under X11.
KDE developer Kai Uwe Broulik wrote a blog post aorund the improved KDE clock handling thanks to Wayland picture-in-picture mode. While experimental though, the Wayland PIP protocol is gated by the KWin compositor with the "KWIN_WAYLAND_SUPPORT_XX_PIP_V1" environment variable.
Those wanting to learn more about this KDE picture-in-picture Wayland effort to benefit their clock and more can find the details via Kai Uwe's blog .
"This allows to "pop out" the stopwatch page or a timer, similar to how it's possible with a video in a web browser. Clicking "return to app" returns back to the page that was popped out."
The KClock support is pending via this merge request .