It's cause by the fact ValEdit sets EO_HOOKKEYDOWN in EditorOptions whenever an Editor is created.
Thiis is needed because otherwise Ctrl+Del will not be passed to the StringGrids KeyDown an we will never see that key combination (which we must act upon).
The downside, as it now appears, is that with EO_HOOKKEYDOWN, the StringCellEditor now has an OnKeyDown event: TCustomGrid.EditorKeyDown. This procedure then handles the left and right arrow by moving to the left or right cell...
I'm unsure if this is intended to work like that, Jesus Reyes should know.
Bart