In the "Open File" dialog, you are not able to select a directory for comparison.
You should be able to select a file OR directory.

NOT WORKING (FOR DIRECTORIES)...

When you select a directory in the "Open File" dialog, and click the Open button, the "Open File" dialog does not close, and the directory path is not entered in the "Source" or "Destination" text fields of Kompare's "Compare Files or Folders" dialog.  Instead, the selected directory opens in the "Open File" dialog.

WORKING (FOR FILES)...

When you select a file in the "Open File" dialog, and click the Open button, the "Open File" dialog closes, and the file path is populated in the "Source" or "Destination" text fields of Kompare's "Compare Files or Folders" dialog.

EXPECTED BEHAVIOR (FOR FILES & DIRECTORIES)...

When you select a file or directory in the "Open File" dialog, and click the Open button, the "Open File" dialog closes, and the file path or directory path is populated in the "Source" or "Destination" text fields of Kompare's "Compare Files or Folders" dialog.


$ uname -a
Linux PS001 4.4.0-53-generic #74-Ubuntu SMP Fri Dec 2 15:59:10 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

$ lsb_release --all
No LSB modules are available.
Distributor ID: neon
Description:    KDE neon User Edition 5.8
Release:        16.04
Codename:       xenial