The observed behavior is caused by the fact that the controls, which appear in the edit row, take up more space than the text in non-edit mode. As a result, the Grid's MasterTableView expands. There are two alternative solutions:
1) Ensure that column widths are initially large enough, so that when the edit row appears, there will be enough space for the edit controls (textboxes, comboboxes, etc).
2) Subscribe to the ItemCreated event of RadGrid and change the widths of the controls in the edit row. (we will implement this in our online example)
Let us know if you need further advice.
the Telerik team