Then, and BatchUIUpdates, previously only worked for a SYNC refresh: the
calling goroutine blocked in wg.Wait until every scope had finished, and
only then flushed the batch and ran Then. An ASYNC refresh had no such
join point — it dispatched each scope onto its own worker and returned
right away — so Then was forbidden (it would have run before the scopes
finished) and a batch would never be drained.
Give the async path a join of its own. Both paths now register their
scopes in the WaitGroup, and the finishing work — wg.Wait, the batch
flush, and Then — moves into a closure. A SYNC refresh runs it inline as
before; an ASYNC refresh dispatches it to a worker, so the caller still
returns immediately but the batch and Then run once every scope is done.
Besides lifting the restriction, this makes SYNC and ASYNC differ only
in whether the finishing work blocks the caller, which is what lets a
later commit drop the mode entirely and key the choice off the calling
thread instead.
bfd3b7b47e