Do you mean a flatpak? Flatpaks are notorious for not integrating. Try to see if hidamari is available first as a .deb file in the official repos (uninstall the flatpak first). If that's not it, then it seems that it doesn't work with the Cinnamon desktop, which has control over the wallpaper functions.
I was able to solve the problem. Instead of downloading it from the Software Manager I installed it from the terminal instead.
When I installed it from the software manager it didn't download one of the packages (org.gnome.platform/44) but when I did it from the terminal it did.
Can you elaborate on that? I am a new Linux user and read a lot about flatpaks in the last week. My impression was, that while space demanding, flatpaks are super easy integrated and that's why they are so succcessful?
It depends, since flatpaks are sandboxed, they don't have access to anything by default. The developer can set defaults for what their app is allowed to access and the user can also manually change that. There's also portals, so you can give them access for a file once (e.g. when opening in a file in an app) or allow them to see your screen and so on. There's still a lot of things that don't have portals tho, so flatpaks don't have access to that.
It's not false information. There are a lot of system-oriented things that don't work through appimages or snaps or flatpaks, exactly because they're sandboxed.
Love it when people speak with authority and are confidently incorrect. Eugenia is right.
You could potentially use flatseal to grant the flatpak the necessary permissions, and you might find out what those permissions are by looking for other users experiences with the flatpak version.
Or, you find the .deb file and it installs natively without being sandboxed. OR, you can find a PPA repository for it, load said repository and install your software.
But those things require learning a little. Linux rewards self starters who can use a search engine and forums. Hope this maybe points you in the right direction.
But desktop integration works without issues. App stores also use flatpak install directly instead of packagekit, at least on cross-distro desktops like KDE and GNOME. Which works way better.
Regarding "they dont work well for system related stuff", on Unix stuff everything is a file, and especially dedicated apps like a wallpaper switcher can have very specific static filesystem permissions, allowing them to do exactly what they need.
Appimages are totally different, a flawed concept by design and have no installer by default so they often have no integration at all. They are also not sandboxed at all and thus just as unrestricted as system apps, while they have no repo, no updating mechanism, no shared libs and are basically a security nightmare.
I dont know about snaps. Their sandboxing needs Apparmor so it is not cross platform.