Function reserve_clipboard_copy 

pub fn reserve_clipboard_copy() -> ReservedCopy

Claim the Ctrl+C clipboard for text that is not known yet, such as a link the person’s click asks a host for, and copy it once it is handed over (ReservedCopy::fill), as try_clipboard_copy would. Call it inside the click: a browser copies only there, and lets a copy claimed there be filled later. Elsewhere the text is copied when it arrives. Dropping the reservation copies nothing.