Function: markFirstScreenSyncReady()
markFirstScreenSyncReady():
void
Mark the first screen as ready to sync when firstScreenSyncTiming is 'manual'.
The main thread holds the UI control until this is called, so the handover timing to
the background thread (for hydration) is fully controlled by the user. It can be called
from both threads (a background-thread call is forwarded to the main thread), is a no-op
unless firstScreenSyncTiming is 'manual', and has no further effect once called.
Returns
void