How to debug (and fix) icon problems?

On GNOME there are usually programs without icons for the windows. Most such problems are related to the StartupWMClass. But sometimes it’s unclear how to fix it.

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I use GNOME’s looking glass (ALT+F2 → lgWindows) to inspect wmclasses.

Missing icons are usually either due to a doubly wrapped executable (ie. the wmclass erroneously contains a dot prefix) or it simply doesn’t match the .desktop entry at all.

Searching for StartupWMClass on the bug tracker provides examples of how to fix it.

I just encountered a weird case. When I change the StartupWMClass of SafeEyes, the wm class in lg is also changed. Not sure if it’s a wayland problem. And JabRef has icon, but the wm class doesn’t match the StartupWMClass.