Callback can hold one listener that can be queued to execute with emit() method.
The real execution will be done later on the same thread.
Callbacks are queued because this prevents mutability problems when callbacks are called from callbacks
and you are using Rc.
EventSubscription is an owner of the callback (handler).
Event keeps only a weak reference to the callback (handler).
JoinHandle will abort the task when dropped.
size in millimeters
size in range 0..1 - covers whole window area
ObservableCollectionFlatMap.
ObservableCollectionIterator.
ObservableCollectionMap.
ObservableComposite.
Warning!
size in scaled pixels (pixel size defined by user preferences)
if not specified otherwise this is the default unit to measure things