The drain loop's `if (settled) break` needs the run to settle in the window
between two queued frames. A file probe on the concurrent-replies case shows the
queue does reach depth 11, but the wall clock never lands inside that window, so
the branch is not schedulable from a test; a case written to force it passed
without ever executing the line, so it is removed rather than left as coverage it
does not provide. The branch carries a v8 ignore naming what is unreachable.
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