Is it somehow possible to have all groups collapsed by default ?
I'm aware that I can use: DayPilot.Row.groups.collapseAll(); after gantt is loaded, but I want them to be collapsed as default.
You probably mean the Gantt chart which displays the nodes expanded by default.
You can override the default value using row.collapsed property of the data object:
text: "Task 1",
children: [ ... ]
The DayPilot.Row.groups.collapseAll() method doesn't control the tree node expanded/collapsed state but "concurrent event groups" in the Scheduler: