The divider is provided by the underlying MasterDetailPane (a ControlsFX control that extends SplitPane). The MasterDetailPane allows you to set the position based on a value between 0 and 1. So you need to compute the preferred width of the tree table and set it in relationship to the entire width of the Gantt chart. This is the value of the divider position. E.g. the preferred width of the table is 500 pixels and the current width of the Gantt chart is 1000. Then you should call gantt.getTreeTableMasterDetailPane().setDividerPosition(.5).
