What’s wrong?
The issue is that these snaps do not use the icon themes from gtk-common-themes. That snap contains the cursor themes. Application snaps like spotify should connect to gtk-common-themes so they have access to all the common cursor themes. Without those cursor themes, the cursor will fallback to the ugly pixelated X11 one. Depending on what kind of snap it is, this is easier or harder to do. You can see whether a snap uses gtk-common-themes by running snap connections snap-name after it is installed. For cursors specifically, it should include the following line:
content[icon-themes] snap-name:icon-themes gtk-common-themes:icon-themes`
How to solve this
The easiest method is to use the gnome-3-28 extension. GTK apps and all other apps (except 32-bit apps, GTK+ 2 apps and Qt apps) should use the gnome-3-28 extension. However, many apps still use the older desktop helpers, which require you to manually add support for gtk-common-themes.
- Spotify should probably upgrade to the
gnome-3-28extension. However, they might be more comfortable with only adding thegtk-common-themessnap. Since this snap is closed-source, there is not much more we can do except bugging them and providing good documentation. - The KDE snaps should, in my opinion, use the
kde-neonextension. That extension adds support for the icon themes (I added this myself). The KDE Neon team, however, mostly still uses custom tooling which doesn’t automatically add the gtk-common-themes snap. The short-term solution is for somebody to take a look at the custom neon tooling and docs, and fix it so they also use the icon themes snap. All the KDE snaps are open source, so a community member should be able to go through all the snaps and addgtk-common-themessupport. The long-term solution is for someone to reach out to the KDE Neon team and see what needs to be done for them to go all-in on thekde-neonextension.