Comment 251 for bug 11334

X11 was not designed to have a persistent clipboard, deferring this functionality to elsewhere - typically the window manager, or some utility running thereon. X11 was designed to be modular, like everything else in *NIX, and AFAIAC this is a good thing. Consider the issues of resource overheads and security for a persistent plaintext clipboard, and how such a thing should not be a default element of the core of any window system.

Conversely, Microsoft Windows lacks any native primary selection buffer (copy-on-select), and the middle-click-paste function, without using an add-on utility, a fact I found intolerable and deeply frustrating, back in the days I used Windows.

If I had to choose between having a native persistent clipboard, or the copy-on-select/middle-click-paste function of X11, I'd pick the X11 way every time. The clipboard was only ever meant to be transient, so using it like some sort of notepad is broken behaviour, IMO. If you need such persistent notes, install a text editor, and save your notes to a file.

For those who really insist on a persistent clipboard, there are plenty of utilities for that.