Expander with GridSplitter
Once you have used the splitter the grid column widths will no longer be 'auto'.
One solution is to respond to the collapsed event of the expander and reset the width of the first column to auto using column.Width=GridLength.Auto;
As you are resizing the contents of the Expander it may make more sense to place the GridSplitter inside the Expander, and on the outside you just have a normal Grid which makes the cell where the Expander is size to content.