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I have got some applications such as KColorChooser, KBreakOut, MadView3D which are not showing any application icon on the dock when they are active. Only a square with no icon is shown. Also, When I hover over them, instead of the application name, unknown is shown.
enter image description here

I am using Gnome as my desktop environment on Ubuntu 20.04. Why does this happen and how to fix this?

2 Answers 2

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I have solved the problem with the help of this video: https://youtu.be/xZ_M3Q-U-J0.

Here is the summary:
Gnome (and any other Desktop) identify instances of an application and keeps track of them with the WM_CLASS property.
Some of the KDE applications do not provide a WM_CLASS property within them and due to this missing WM_CLASS property, Gnome cannot keep track of the applications and therefore cannot associate the icons with them. Every time a new instance of that same app is created, it shows a new element on the dock.
This problem also occurs in KDE plasma: https://forum.kde.org/viewtopic.php?f=289&t=160452

Solution:

  • Open the application which behaves this way.
  • Open up a terminal and issue the command xprop WM_CLASS. The mouse cursor will change to a + symbol. Click on that application with the + shaped cursor. The WM_CLASS string will be printed out on the terminal.
    enter image description here
  • Copy the .desktop file of that application from /usr/share/applications/ folder to your local directory ~/.local/share/applications/ folder. For me, it was named org.kde.kcolorchooser.desktop.
    Do not modify the original file, because it will get replaced whenever the application is updated.
  • Open the file in a text editor and add the line StartupWMClass=<WM_CLASS> where <WM_CLASS> should be replaced by the string you got from the above command. For me, it was StartupWMClass=kcolorchooser.
  • Save the file and enjoy! The problem will be gone.
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If you are facing issues with the Ubuntu taskbar icons, please run this commands to get rid of taskbar icon missing problem.

sudo update-icon-caches /usr/share/icons/*

sudo apt install --reinstall gnome-control-center
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