Copy/paste has never worked properly in all cases in GNU/Linux, for decades, and this problem is still not fixed.
I find myself with two xfce4-terminal instances, terminal A and terminal B.
Terminal A is at a bash command prompt. Terminal B is in a vim instance, editing some arbitrary file.
I attempt to copy/paste text from B into A, by highlighting the text in B with the mouse cursor. The text becomes successfully highlighted. I use the middle button in terminal A.
The text from terminal B should be copied into terminal A.
The previous entry in the clip buffer gets dumped in terminal A instead.
Get out of vim and use less or similar command line tools, copy the text without being in vim. Then it works as expected.
Is there a better fix?