Hi Ken, 

Could it be that you mixed up kdiff3 https://apps.kde.org/kdiff3/ with KDirStat, while we were talking in the "Community News for June" show?

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QDirStat is based on that same code from the original KDE 3 KDirStat of 2006. It's an 80% rewrite using a lot of newer Qt technologies. And there was a lot of cleaning up that old code base that had been long overdue. 

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If so, QDirStat is also interesting, it comes with nice features like 
Package manager support:
- Show what software package a system file belongs to.
- Packages view showing disk usage of installed software packages and their individual files.
- Unpackaged files view showing what files in system directories do not belong to any installed software package.

And is just an apt install away :-)

However, you helped me anyway!
Because, while in KDE Dolphin's kdiff3 integration can only compare two files in the same folder, I can use it in the terminal with paths to the files like:

kdiff3 /home/reto/abc.txt   /media/usb/abc.txt

Br,
Reto