A request is now marked superseded before the previous one is
cancelled, so a query that completes synchronously on cancellation
returns quietly instead of clearing the page and surfacing a stale
cancellation error. NestBrowseSession.IsLoading follows only the latest
request, and the dialog's Previous/Next and Loading text use it rather
than a count that included superseded requests. Each request disposes
its own cancellation source when it ends.
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