Enabling and disabling edit mode.
Make an Edit Mode toggle so that way arrow keys only become effective inside cells, and no longer move between cells. Would be very useful and time saving when transferring data to be able to use the arrow keys to move back a few characters without having to use a mouse to select a spot within the cell or re-select the cell.
Gareth Hayter commented
When you're in 'partial' edit mode inside a cell, pressing the arrow keys will end edit-mode and move you between cells, as you've described. To get the behaviour you want, just press 'F2' and you will enter full edit-mode - now pressing the arrow keys will move your caret position inside the formula.