Desktop state is neatly organized using freedesktop standards. Since components are so modular, it's trivial to replace or modify any of them using any client program you wish.
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This is peak "Year of the Linux Desktop" behavior
What if your background is a slideshow? Or what if it changes depending on what time of day it is or what the system's current theme is? Or what directories it should search for background images in? Even though COSMIC does not yet support all these features, they're setting a foundation for COSMIC to be expanded upon in the future in a way that's entirely unique in the Linux space rn.
Ya that's totally fair. I just thought it was funny that the desktop background was one of the only two apps that appear to have state on your system.
Speaking of the Cosmic background, since they are already rendering it with the GPU they should let you specify a custom frag shader for it. Then you could have some really wild desktop eye candy.