I'm going to merge this immediately. I just added you as reviewers @troycurtisjr @gornostal to let you know. This wasn't possible to do before because of the (now fixed) focus issue.

Posted from KDE live CD (testing):

Update:

The current KDE implementation (now deactivated) didn't work for me, but this did work 100% on both X11 and Wayland without focus issues on Kubuntu with systemd:

1. systemctl --user is-active plasma-kglobalaccel (should output "active". Then we know the service is supported)
2. kreadconfig5 --file kglobalshortcutsrc --group io.ulauncher.Ulauncher.desktop --key "_launch" to check if it has already been set (then leave it).
3. kwriteconfig5 --file kglobalshortcutsrc --group io.ulauncher.Ulauncher.desktop --key "_k_friendly_name" "Ulauncher" && kwriteconfig5 --file kglobalshortcutsrc --group io.ulauncher.Ulauncher.desktop --key "_launch" "Ctrl+Space,none,Ulauncher" (to set)
4. systemctl --user restart plasma-kglobalaccel to apply the change.

This also adds it with the rest of your application shortcuts and adds an icon.

I think that's as good as it gets. To add it as a "default shortcut" instead of "custom" I think you need to make it a native KDE app and use this SDK (example).

You can then open this view directly by running systemsettings5 kcm_keys instead of opening our custom hotkey dialog.

An older thread suggested a more naive way to restart the service, which could be coupled with pidof kglobalaccel5 to feature detect. However given how many issues we had with subprocesses getting killed when ulauncher is restarted I think systemd is the only way.

The hotkey in in another format. It looks just like the "label" format you can get from the gtk accelerator one: Gtk.accelerator_get_label(*Gtk.accelerator_parse(hotkey)), but it's not 100% the same (this would cover most of it though: replace("Super", "Meta").replace("Escape", "Esc").replace("Page Up", "PgUp").replace("Page Down", "PgDn") for example (see more here). I don't feel great about converting without knowing for sure that this will work for every possible scenario and not break the system. However, maybe we can just set it to Ctrl+Space and run systemsettings5 kcm_keys so the user can set it to what they actually want.

Old post

Did some testing on KDE, and found that this didn't actually work (based on an outdated article with fake date saying June this year).
Also, focus stealing by default is enabled (level 1 - low) and you have to disable it.

It can be disabled programmatically with
kwriteconfig5 --file ~/.config/kwinrc --group Windows --key FocusStealingPreventionLevel 0. We shouldn't do that of course, but one quickfix would be to disable it before launching ulauncher every time, then restoring the previous setting (which can be read with kreadconfig5 --file ~/.config/kwinrc --group Windows --key FocusStealingPreventionLevel).

Also, the empty space has an icon now, so I guess that's what it's supposed to have:

Related article about KGlobalAccel says it's only X11 anyway: https://blog.martin-graesslin.com/blog/2015/06/global-shortcut-handling-in-a-plasma-wayland-session/ Need to test this again with kubuntu wayland sudo apt install plasma-workspace-wayland (and log out/in again and choose Wayland session)

Great stuff @friday!
And thanks for the very detailed PR description, btw.

Have tested and fixed the KDE and XFCE implementations now, and they work great (after pushing fixes). So we have great support coverage already. I am pretty sure Budgie, Panteon and Cinnamon uses the same way as Gnome, but with different gsettings paths, so maybe I will add those or some of them. I won't add support for more than those though. Should be easy for contributors who are on smaller desktops to contribute.