Re: [w3c/clipboard-apis] Defer OS clipboard read from clipboard.read() to ClipboardItem.getType() (PR #248)

@shwetabin commented on this pull request.



> @@ -918,6 +934,60 @@ url: https://storage.spec.whatwg.org/#obtain-a-storage-key-for-non-storage-purpo
 
     1. If |representation|'s [=representation/MIME type=] is |mimeType| and |representation|'s [=representation/isCustom=] is |isCustom|, then:
 
+     1. If [=this=]'s [=ClipboardItem/clipboard change count at read=] is not null, and the current [=clipboard change count=] is not equal to [=this=]'s [=ClipboardItem/clipboard change count at read=], then [=reject=] |p| with an {{"InvalidStateError"}} {{DOMException}} in |realm|, and return |p|.
+
+      Note: This ensures that stale data is never returned. If the [=system clipboard=] contents have changed since {{Clipboard/read()}} was called, this MUST fail rather than returning data that does not correspond to the current [=system clipboard=] state.
+
+      Note: A null [=ClipboardItem/clipboard change count at read=] indicates that this {{ClipboardItem}} was constructed directly by the author (e.g., via {{ClipboardItem/ClipboardItem(items, options)}}), not obtained from {{Clipboard/read()}}. In that case, no stale-data check is needed because the data does not originate from the [=system clipboard=].
+
+     1. If [=this=]'s [=ClipboardItem/clipboard change count at read=] is not null, then:
+
+      Note: This branch covers {{ClipboardItem}}s obtained from {{Clipboard/read()}}, for which the OS clipboard read is deferred to this point. To ensure the read happens at most once per type — even if {{ClipboardItem/getType()}} is called multiple times concurrently for the same MIME type — the resulting {{Promise}} is cached in [=this=]'s [=ClipboardItem/representations with resolvers=] map, keyed by a normalized type string so that `"text/html"` and `"web text/html"` are tracked independently while differently-cased spellings of the same type (e.g., `"text/HTML"` and `"text/html"`) collapse to a single entry.
+
+      1. Let |key| be |mimeType|'s [=MIME type/essence=]. If |isCustom| is true, prefix |key| with `"web "`.
+
+      1. If [=this=]'s [=ClipboardItem/representations with resolvers=][|key|] [=map/exists=], then return [=this=]'s [=ClipboardItem/representations with resolvers=][|key|].

In ClipboardItem::getType, lazy (read()-created) items cache the resolver in representations_with_resolvers_ and return the same promise for repeated getType() of the same type; eager (author-constructed) items return a fresh .Then(...) promise each call. The cache is what guarantees the OS clipboard is read at most once per type

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