When clicking a file or folder, there is no KDIFF3 integration in the context menu that allows you to either 'save file for later comparison' or compare files with kdiff3 when that plugin is installed. This has always been vital to konqueror. When rt-clicking a file or folder, the KDIFF3 context menu entry should appear and provide the traditional functionality is has provided. It is currently missing.
Well, that's not a bug in Konqueror. It's because kdiff3 hasn't been ported to KF5 yet.
KF5 port has been completed.