@ggenois I didn't change anything, and there are no themes involved ? not sure why you think it's related at all to that link.

The KDE app is trying to show itself in some way that is broken outside of KDE. Typically the Qt based stuff tries to find a theme based on the current desktop/OS/etc, but here it fails badly.
The fact that the window decorations are in the wrong place leads me to think there might be some "gtk integration plugin" I am missing.

Anyhow, it just looks silly.