I'd like that when I select a cell to edit (a big cell with text), it deselects contents and sets cursor at the beginning of the cell. I tryed with textbox properties like selectionstart but it doesn't work.
I'm trying to emulate an email software where I answer a previous message and the answer text is in a cell , with the previous message at the bottom of the cell
When and how did you try to set the SelectionStart/SelectionLength properties of iGrid's TextBox object? If it was an event handler of the RequestEdit event, then you did this before iGrid selects the text in the editor and your assignments did not work because of that.

I think you need a timer you start in an event handler of the RequestEdit event and then set the TextBox selection properties in the Tick event of the timer. In this case your settings will be done after the editor is activated by iGrid, but before the user gets the ability to enter text.