Comment # 3 on bug 977040 from Wolfgang Bauer

(In reply to 6tr6tr R from comment #2)
> This only partially fixed the issue. YAST now shows the icon, but my 3rd
> party apps like IntelliJ still show the generic one. How can I get this
> working too?

No idea, depends on why it is not working.

What apps exactly are having this "problem"?

You said that they do have a (different) icon in the application menu.
Is this also the case for those that are still affected now? Do they have an
icon in the window title bar?
It might not be a "bug" in Plasma at all. If the taskmanager doesn't get/cannot
find an icon, it has to use a generic one obviously...

Where did you install IntelliJ from?
I installed the package intellij-idea-community-edition I found on
http://software.opensuse.org/package/intellij-idea-community-edition (I suppose
that's what you mean with "IntelliJ"?), and it does show its icon in the
taskmanager here.

Maybe try to delete the icon cache (~/.cache/icon-cache.kcache) and run
"kbuildsycoca5 --noincremental" as user, or try with a fresh user account.

If that doesn't help, it would probably be a good idea to report it upstream at
http://bugs.kde.org/ (with details on what applications are affected).
The developers who actually wrote the taskmanager (and conceived how it should
behave) probably have better ideas what might be going wrong and might even be
able to improve things...

And it's definitely not an openSUSE packaging issue anyway.