To stretch the DataGrid to more than 100% of its parent, the width attribute of the mendixDataGrid class could be set to the desired width. This could be both a static width or a percentage of its parent.
.mendixDataGrid {
width: 2000px;
}
This will set the width of all DataGrids to 2000px. If this should not be applied to all DataGrids, you could add a class to your custom theme, which defines the desired width. This class could then be set on the particular DataGrids in the Modeler.
Bear in mind that column widths in the Modeler will still be relative to the DataGrid. All elements in the form with no width specified or a width of 100%, will also stretch to the DataGrid's width.
Edit: To achieve the effect you are looking for, you should instead specify the width on the mendixDataGrid_gridTable class (with a preceding custom class, to avoid all DataGrids get stretched):
.inModelerToDataGridAttachedClass .mendixDataGrid_gridTable {
width: 3000px;
}
Also add the following css to scroll on the DataGrid its content instead of on the form:
.mendixDataGrid_tablePane {
overflow-x: auto;
}
Edit 2: For 3.2 onwards, see this forum question: https://forum.mendix.com/questions/3469/New-functionality-in-data-grids