I installed Spotify client using the method from their official site. When I run it, however, a low-res icon pop out on the dock and is used to maximise/minimise the app instead of the original hi-res icon. Any idea why this happen and how to fix it?
For some reason, Plank is unable to match the Spotify window to the original launcher's .desktop file. It's using a "fallback" icon from the window itself. This seems to be something that happens fairly often on cross-platform, non-native apps like Spotify, Chrome, Minecraft, etc.
I haven't tried it myself, but supposedly you can fix it by modifying Spotify's .desktop (stored in /usr/share/applications) by changing the line
Exec=spotify %U to
Be aware that this may affect Spotify's ability to accept arguments from the command line or other features involving opening external files. For more information, you can see this explanation of what