TestScreenArtReprobeWindow failed under -race -shuffle in CI: the probe
slot is process-wide, and a probe an earlier test started against its
own (now closing) server still held it when the next test asked for a
re-probe, so that request was dropped and the stale avatar never
re-probed. The leftover probe could also write its result over the
state the next test had just reset.
Probes now run in a WaitGroup that resetScreenArt waits on, so a reset
between mock servers always starts from an idle probe slot.
MHSanaei
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