There are lots of ways to move data between or within graphical windows! But the list is shrinking

• clipboard - cltr-c / ctrl-v
• drag and drop - mouse selection, mouse drag
• primary - mouse selection -> middle click (or shift+ins); moves text cursor
• secondary - ctrl+select (alt+select in emacs); secondary doesn’t move text cursor. largely forgoten. motif, patched gtk; not in qt, tk, mainline gtk, etc
• cut-buffer - deprecated everywhere. set by application using X11 window property for state

https://en.wikipedia.org/wiki/X_Window_System_selection

It’s in emacs! Using alt instead of ctrl https://www.gnu.org/software/emacs/manual/html_node/emacs/Secondary-Selection.html

You’re describing the primary selection. The article’s about patching secondary selection back into gtk3 (c. 2015?). It’s ctrl+select followed by ctrl+middle to paste as yet another clipboard. This has the unique and useful property that neither selection nor paste changes the text cursor’s position.

A more in-depth look at secondary selection is in the embedded video which has it’s own write-up on https://www.cs.man.ac.uk/~lindsec/secondary.html

[I suspect primary-secondardy selection is not a confusion you have, but one a casual skip-the-article-direct-to-comments reader might have]

And the keyboard shortcut for primary clipboard paste (Shift+Ins) is already gone from gtk.

• https://gitlab.gnome.org/GNOME/gtk/-/issues/352 (closed)
• https://bugzilla.mozilla.org/show_bug.cgi?id=1070518 (open)
• There’s a nice Q/A on stackexchange: “Why does Shift+Insert paste from CLIPBOARD in some applications and PRIMARY in others?”