Some KDE themes display drop shadows on the window's border. It is good for both aesthetic and user experience, but there could be cases where it's not desirable, such as taking a screenshot of a window panel.
Breeze is the default KDE theme for Linux distributions such as Kubuntu. While this guide is specific to Breeze, the location and option of how to disable panel shadow for other KDE themes is generally the same.
The panels could be different depending on the theme, and the option to change the drop shadow might not exist at all. This example is based on the Breeze theme.
Comment anonymously. Login not required.