The default run command for kdiff3, which I can no longer tell you because wayland somehow wiped my clipboard, won't start kdiff3 in a wayland session. It runs just fine with just "kdiff3".
Be advised your distro's default version is unsupported and is no longer maintianed. Please tell Arch to upgrade there package to use https://anongit.kde.org/kdiff3.git. If rebuilding from source here does not solve the problem I will look into it.
Please make a proper release that we can update to. Packaging from git has several problems, the main one being missing localization
Will do should within the next month depending on how fast things go. Translations are imported automatically as part of this process. Also I did have a look at this and removed the default parameters which appear to be unneeded.
Git commit 89c6c8ca1986dd449aeaf8edc194c3daa6c02335 by Michael Reeves. Committed on 30/07/2018 at 06:04. Pushed by mreeves into branch 'master'. Fix 395686, remove uneeded default params FIXED-IN:1.7 M +1 -1 src/org.kde.kdiff3.desktop https://commits.kde.org/kdiff3/89c6c8ca1986dd449aeaf8edc194c3daa6c02335