Gantt dependencies can be created and destroyed, but cannot be edited. If you want to edit a dependency, you will have to destroy it and create a new one. Destroying a dependency is done by selecting it with the mouse and pressing the Del key on the keyboard.
The gantt widget only has logic to move related parent and child tasks, in order to keep the parent (summary) tasks up-to-date and accurately representing their children's state (overall percentage of completion, earliest start and latest end). When it comes to dependent tasks, since you can create unlimited relations between tasks, creating circular references etc., we chose to leave this functionality to be implemented manually by the developer.
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