Documentation

Using MVar for synchronizing because it guarantees FIFO wakeup which will help prevent old updates overriding new updates. The constructor is hidden to maintain the expectation that the last two member will keep the ShimCallbacks inside the Scene alive. Before removing Subject from a container, use prolong to store the prolonging IO action "nextRenderedListener" action handler, so that the ShimCallbacks is garbage collected only after the child widget is no longer rendered.

Creates an IO action whose existence will keep the lease alive. Running it has no side effects.