In your code, you should check your output window to see if any exceptions are being thrown that is causing this behavior.
Here is a sample that seems to work OK for me. You can select a value from the dropdown, and then when you leave the new row, it will add a row at the bottom of the grid. The combobox also drops using the width of the cell, and there is no space on the left except the column header. If you cannot spot what is different about your code and this sample, then if you will post a working sample showing the problem, we can try to debug it here. I am using 2.1.0.9.
GGCAddNewCombo_7532.zip