Here’s a working example of your code. It only took minor changes to get it working. I've added comments where I made changes.
# The original example had the grid way off to the right. This was causing
After your last e-mail I got confused (you got the code working, mine didn't work) and investigated until I discovered that the horizontal scroll bar doesn't cause scrolling. The cells were truncated and the expansion 'buttons' were not visible.