[w3c/clipboard-apis] Browser inconsistency on reading the data store after a microtask in a paste/drop event listener (Issue #255)

GaurangTandon created an issue (w3c/clipboard-apis#255)

**Browser inconsistency**
 
- In a 'paste' event listener, Chrome (and Safari) allows reading the contents of event.clipboardData _after_ a microtask await. Firefox does _not_ allow it.
    - Had filed (old) crbug for this: https://issues.chromium.org/issues/40913972
- In the 'drop' event listener, all three browsers allow reading the contents of event.clipboardData _after_ a microtask await.

**Spec**

[The spec for clipboard events](https://www.w3.org/TR/clipboard-apis/#privacy-events) states the following. It suggests that Firefox's more conservative behavior for 'paste' is probably correct.

> -   Objects implementing the `[DataTransfer](https://html.spec.whatwg.org/multipage/dnd.html#datatransfer)` interface to return clipboard data _must not_ be available outside the ClipboardEvent event handler that the data is provided to. 
> -   If a script stores a reference to an object implementing the `[DataTransfer](https://html.spec.whatwg.org/multipage/dnd.html#datatransfer)` interface to use from outside the ClipboardEvent event handler, all methods _must_ be no-ops when called outside the expected context.

[The spec for DND event](https://html.spec.whatwg.org/multipage/dnd.html#the-dragevent-interface) states the following. It's not clear to me whether this allows or disallows reading the contents after a microtask.

> Dispatch event at the specified target element.
> Set the drag data store allowed effects state to the current value of dataTransfer's effectAllowed attribute. (It can only have changed value if e is dragstart.)
> If dataDragStoreWasChanged is true, then set the drag data store mode back to the protected mode.
> Break the association between dataTransfer and the drag data store.

**Impact**

This leads to developer confusion and cross-browser bugs, like here: https://github.com/strictdoc-project/strictdoc/issues/3254

**Notes**

This issue is related but stale: https://github.com/w3c/clipboard-apis/issues/50


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