The floating events are enabled by default. It works as follows:
1. Events that start before startDate will have *_event_continueleft class. This doesn't change with scrolling.
2. The float div (marked with *_event_float) is added to events that start before the current viewport, but only if the scrollbar position is greater than 0. If you scroll back so that the event is fully visible in the current viewport or if the scrollbar position is 0, the float event is removed.
So you might want to define styles for *_event_continueleft to indicate that the event starts before startDate.
Please let me know if it doesn't work as expected.