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From the Browser’s viewpoint, 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?
It’s not like there’s that much of a difference, as far as the Browser is concerned.
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.