I would like to show visual feedback during Drag&Drop where drop for an activity is allowed.
What is the best approach to mark certain areas of a row (complete row or just some timeslots) with a special color during Drag&Drop?
Add a system layer to GraphicsBase (see GraphicsBase.getBackgroundSystemLayers()). Inside the layer you can retrieve the currently dragged activity by calling GraphicsBase.getDragAndDropInfo(). The drag and drop info contains the dragged activity (bounds), the time interval where the dragged activity would be dropped, the row over which the activity is currently hovering.