The onRowHeaderResized event is called when a users resizes the row header splitter (on drop). Only applicable when row header scrolling is enabled.
DayPilot.Scheduler.onRowHeaderResized(args);
The new row header width is accessible using rowHeaderWidth property.
DayPilotdp.onRowHeaderResized = function(args) {
console.log("New row header width: ", dp.rowHeaderWidth);
};