Raised before the viewer writes to the clipboard: when the selection is copied (with the copy hotkey, CopyToClipboardAsync() or CopyAsMarkdownAsync()) and when a code block's copy button is clicked. A handler can change Text or set Cancel. Not raised when there is nothing to copy. Bubbles up the visual tree.
public event EventHandler<CopyingToClipboardEventArgs>? CopyingToClipboard