In the current implementation of RadGanttView
such customization is not available.
When the user expands or collapses a GanttTask remeasure or rearrange of visible containers is not performed.
I've prepared a sample project that demonstrates the switch between summary container and event container for each SummaryGanttTask. In it I modify the SummaryContainer control template adding another grid ( same as the grid in the EventContainer ) and hide/show the correct grid depending on the value of IsExpanded of the CustomGanttTask. In the RadGanttView.ExpandCollapseService.HierarchicalCollectionAdapter.CollectionChanged
event handler I modify the value of IsExpanded for the CustomGanttTask. This event is called each time a user expands/collapses a GanttTask.
Unfortunately, this approach works when the user scrolls out of the collapsed task and returns back, because in this scenario the UI is remeasured and rearranged. It will also work correctly with a set RadGanttView.InitialExpandBehavior.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.