When you want to control that when a user starts a selection within a position (resource) in the Planner, the select-drag will be restricted to the position where the select-drag started, a new property is available:
Planner.SelectPosRestricted: boolean;

Set this to true and even when columns or rows for resources are small and sometimes difficult to keep the mouse in the same resource during long select-drag operations, the selection will remain within the originally selected resource.