> Until you close the app where you selected the text and now your clipboard is empty.
Ho, I used it for years, and you made me discover that strange property. Interesting.
> Or until you click somewhere, original app loses selection and your clipboard is now empty.
Yes, it's a very volatile clipboard, but if you expect your clipboard to have a lifespan greater than 5 seconds, you're probably doing something wrong.
You're both correct. Yes, it acts as a clipboard, for all practical purposes. But also it's a terminology thing.
If you'd open ICCCM, it uses name "CLIPBOARD" for a different selection atom: https://www.x.org/releases/X11R7.6/doc/xorg-docs/specs/ICCCM...
So in X11 the term "clipboard" (in a technical, not colloquial sense) is reserved for a different selection. And thus primary selection is not a clipboard.
Or until you click somewhere, original app loses selection and your clipboard is now empty.