Automatically putting highlighted text into the clipboard is an old, old Unix convention. I personally hate it, along with the middle-click to paste action. AFAIK neither of those behaviors can be disabled, and I find them both extremely surprising, unintuitive and harmful. I guess some people like it, but if I could turn all that off, I'd be thrilled.

Manually highlighted text going into the selection buffer for quick middle-click pasting is great, and makes me much more efficient at manipulating text on Linux than Windows. If you don't like it, you can just ignore it: don't click the middle mouse button on text areas. The selection buffer is independent of the explicit cut-copy-paste clipboard (if you have some program running that's trying to synchronise them, you can turn that off).

Yeah yeah, it's totally a preference thing. I can't ignore it, because I use the scroll wheel a _ton_ (have you ever used the middle-click autoscroll thing in Firefox? it rules). So sometimes the scroll wheel gets accidentally clicked and I paste some junk into the middle of the document I happen to be scrolling through. I'd much rather have a strong, confirmative action like pressing Ctrl-C and V to interact with the clipboard, rather than overloaded functions like highlighting text or interacting with the scroll wheel. It sucks, but after 10+ years on X11, I've mostly gotten used to it.

This feature predates scrollwheels multiplexed with middle buttons. You could source a scrollpoint mouse to split the behavior. The "modern" ones with the large saddle are a nicer experience than the laptop erasers.

Maybe get a better mouse? My scroolwheel is harder to click than other buttons.

Or you could always disable the middle button:

  xinput set-button-map <input> 1 0 3

i have a habit of selecting text i'm reading. auto-copy into the clipboard totally wrecks my productivity.

It goes into the selection buffer, separate from and independent of the clipboard.

It's a feature I absolutely depend on, I can't even imagine being productive without it.

At any rate, if you use GNOME, you can disable it. Open Tweaks, go to "Keyboard & Mouse" and disable "Middle Click Paste"

Oh trust, me I've tried. That setting only affects Gnome/GTK applications. Some applications that use other toolkits, or no toolkit, have direct support for middle-click paste, which then can't be disabled at all. It's just a platform convention I dislike; I know it won't be going away.

XMousePasteBlock is apparently effective.

It keeps the primary clipboard clear, so it should also work for non-GTK applications.

https://github.com/milaq/XMousePasteBlock

i actually like middle-click paste, but there's no denying that having two distinct OS-level copy/paste buffers with subtly different semantics is a rough edge.