Spawning a centered topmost window that doesn't steal focus

For a Workrave-style “You need to take a break” popup, I’d like to spawn a window in the center of the screen, raise it to the top, and not have it steal focus.

What are my options? I vaguely recall reading somewhere that due to Wayland limitations centering and pinning are no longer supported. However, Workrave still seems to be able to center itself under Wayland. I also know that workarounds like gnome-shell-extension-firefox-pip/extension.js at main · bennypowers/gnome-shell-extension-firefox-pip · GitHub exist to pin windows.

Correction about centering: it appears that I had the center windows option enabled in the GNOME Tweaks tool, so that’s probably why Workrave was able to do it.

Still, I’m curious about any possible workarounds for centering, pinning and focus stealing avoidance.