Please refer to the attached sample project. The ResizeColumnsProportionally.gif illustrates the correct behavior of columns resizing. However, if you make a certain column with fixed size, the columns resizing is not proportional any more. The ColumnsWidthNOTAdjustedProportionally.gif demonstrates better the wrong behavior.

Workaround:  handle the SizeChanged event and adjust the columns' width property programmatically in order to obtain the desired widths.