would there be any difference if the webpage has a JS button to put something in the clipboard, or it having code running in the background that puts things into the clipboard at page load?
Clicking a button shows user intent, whereas a page load doesn't. No user expects loading a page to overwrite their clipboard, but every user that clicks a "Copy to Clipboard" button does expect it.
Clicking a button shows user intent, whereas a page load doesn't. No user expects loading a page to overwrite their clipboard, but every user that clicks a "Copy to Clipboard" button does expect it.