[torgard]

You can set clipboard.autocopy to false in about:config to disable this. It breaks the example, at least.

EDIT: I thought the behavior was preserved for textareas, as the comment box here still copied on select. But it suddenly disabled it here too. Restarting the browser is probably a safe bet :)

[torgard]

I should say, doing this only mitigates the middle-click-paste pastejack exploit. The underlying issue - JAVASCRIPT CAN ALTER WHAT YOU HAVE SELECTED AT WILL AND WITHOUT DETECTION - remains, and I believe we'll see other exploits pop up from this.

Say you're about to copy a crypto wallet. You have it selected, and are about to press CTRL-C to copy it. It is entirely possible for malicious code to detect that. And, as it turns out, it's possible for it to change the selection to a different (invisible) string, right as you press CTRL-C.

[prima-facie]

This is very useful, thanks. With a clipboard manager like Clipman you can tick 'Ignore mouse selections'.