Hi Martin,
The current behavior of the TreeGrid Control for keyboard navigation is, we can navigate between the edited cells using ‘TAB’ and ‘SHIFT + TAB’ keys, please find the below sample for your reference,
http://mvc.syncfusion.com/demos/web/treegrid/treegridkeynavigation
But if we press down or up arrow in the editable state it will navigate inside the selection string.
Also the behavior in the MS Excel is that if we type a new value and press down, it will navigate to the cell below, but if we edit an existing value and press down arrow, it will navigate along the last character in the same cell. Is this your exact requirement same like in MS Excel ? Can you please share more details regarding this requirement to provide you an appropriate solution.
Regards,
Mahalakshmi K.