Manipulating `ClipboardEvent.clipboardData`

• @testing-library/user-event version: v14.0.0-beta.11

Relevant code or config

document.addEventListener('copy', e => e.clipboardData.setData('text', 'bar'))

document.addEventListener('cut', e => e.clipboardData.setData('text', 'bar'))

document.addEventListener('paste', e => e.clipboardData.setData('text', 'bar'))

Problem description:

When double-checking #862 and #866 , I discovered that accessing/manipulating ClipboardEvent.clipboardData is limited.

(In Firefox:)

The data written to Clipboard is independent from the clipboardData on the cut/copy event. Manipulating has no effect.
Also the handler cannot read the clipboard data.

The data pasted from Clipboard is readable on the paste event. But manipulating it results in a DOMException.

Suggested solution:

Check consistency across browsers. If it is, implement API accordingly.

The issue provides no file or test path. Start by reproducing the copy, cut, and paste handlers in the relevant browser environments and comparing clipboardData behavior; done means the supported behavior is consistent across browsers and covered by regression tests.