Now, KDE apps typically do not use client-side-decorated headerbars for their header areas like GNOME apps do. Instead, we generally hew to the traditional arrangement of a titlebar, menubar, and toolbar. The titlebar is “server-side” because it’s drawn by KWin, our window manager. Everything below the titlebar–such as the window’s menubar, toolbar, and content view–are drawn by the window itself; the window being a “client” of the window manager. Hence, “client-side”.
KDE’s approach is so much better and more sane than the CSDs in GNOME. CSDs have wreaked havoc in the world of GTK desktops, with Xfce in particular suffering hard due to its use of Xfwm, causing a giant rift between the looks of Xfwm and the CSDs of many GTK applications. The main issue here is that a title bar is a title bar for a reason – I don’t want it littered with buttons and other widgets that belong to the application, not the window.
I guess I’m just getting old.