In KDE4, there was a spacer element, which can be used to have a little bit of space i.e. between the close button and the maximize/minimize buttons. With KF5, there is no such element in the button settings for the window decorations. If it's dependend on the actual decoration, i am using the "plastik" theme. Reproducible: Always
*** This bug has been marked as a duplicate of bug 348393 ***