Today's cursed X Windows System knowledge is about how X has two cut and paste systems and they don't interoperate, because after people used the first system for a while they realized it had limitations and was kind of a bad idea but of course it couldn't be replaced. Do all programs support both? No, of course not.
Reading material: https://utcc.utoronto.ca/~cks/space/blog/sysadmin/XCutAndPasteHistory
https://www.uninformativ.de/blog/postings/2017-04-02/0/POSTING-en.html
https://utcc.utoronto.ca/~cks/space/blog/unix/XtermModernCutAndPaste