When you launch Kompare on its own (not by clicking on a patch file in Dolphin), you see a welcome window. And that window does not allow you to choose a single diff/patch file.

STEPS TO REPRODUCE
1. Open kompare from start menu
2. Observe the window you see 

OBSERVED RESULT
The window shows you an interface (on the Files tab) that allows you to choose file 1 and file 2 to compare them. But there is no way to open a patch file itself.

EXPECTED RESULT
Ability to choose a diff or patch file on a separate tab at least.

ADDITIONAL INFORMATION
This window that is shown at start is actually a dialog that is shown when you choose menu "File" - "Compare Files". But there is also "File" - "Open Diff" entry.

I suggest to add a nice welcome window, like for example in Ark, saying "Welcome to Kompare" and has two buttons: "Compare Files" and "Open Diff". And that dialog window for several files to be only shown when user actually selects "Compare Files".

SOFTWARE/OS VERSIONS
Kompare: 4.1.22120