It still works for me in Debian stable, with either GNOME 3 or MATE. I'd be sort of surprised if both Fedora and Ubuntu disabled it by default but it's possible

It still works for me in Debian stable, with either GNOME 3 or MATE. I'd be sort of surprised if both Fedora and Ubuntu disabled it by default but it's possible. One thing to remember is that the middle-click clipboard it's a separate clipboard from the Ctrl-C/Ctrl-V one. It's called the "primary selection", and as the name implies, it's updated if you select something new or even unselect the old selection. And nothing automatically synchronized with the Ctrl-C/Ctrl-V clipboard, so if you select something, copy it, and unselect it, middle-click won't work to paste any more but Ctrl-V (or Edit | Paste) still will. If you want them synchronized you can use two autocutsel processes to do this (autocutsel synchronizes between either the "primary selection" or the "clipboard selection," and yet another type of X11 clipboard called the "cut buffer", so you need two to sync each selection with the cut buffer). https://blog.nelhage.com/2008/09/autocutsel/ https://blog.nelhage.com/2008/09/autocutsel/ describes how to do it. You might have had something like this in your RHEL 6 dotfiles that didn't get copied over to your new system.