When I resize the last column to the right of the grid it doesn't show how large I've made the column per se but at least it allows me to resize the column. All the grid examples in the explorer don't work the same way. I can only resize the last column by dragging the vertical scrollbar's width to end of the grid, scroll, drag to end, scroll...etc.
I don't think I can do any better of an explanation. The example link I provided shows how I want resizing to work on the last column (in an effort to be as clear as possible, I mean the column that is furthest to the right side of the grid).
The page you linked here uses Ext JS. The resizing in GXT is done in the ColumnHeader class. There is a GridSplitBar class which you will need to extend. There are probably more changes than this required.