This is a design decision, aimed to improve usability of the control, and was made after we received feedback from many clients using the gantt widget. When you are editing the start date inside the Treelist cell, you immediately update the task. So if for instance you want to move a one day task to start one week later, and want to first update the start by one week, an update will trigger and the task will have broken dates. This is of course handled by the validation, but now you can't move the start with the timespan you need, and have to first move the end, and then the start. In the popup editor this is not needed, since a task update is triggered only after the save button is clicked.
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