The Mark tool (#3081/#3277) captured the preview via the *active* iframe. For
URL-load previews — decks especially — the active frame is the bridgeless URL
iframe, while the snapshot bridge lives only in the (mounted but hidden) srcDoc
transport frame. So Send on a deck timed out and showed 'Could not capture the
preview. Try again to avoid sending only ink.'
Snapshot the srcDoc-render-mode frame instead (capture mode already keeps it on
full content, so it carries the bridge), with a short retry while it finishes
swapping to full content. Falls back to the active frame for the non-URL-load
case where they are the same.
Red spec: PreviewDrawOverlay.test 'snapshots the srcDoc bridge iframe, not the
visible URL-load frame' fails on main (targets the URL frame), passes here.
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