Master-detail datagridviews :cannot prevent user from leaving invalid detail cell

[nina4ever]

in my WinForms app. i have master-detail DatagridViews.
i have a RowValidating and CellValidating event handlers on detail grid.

if detail cell/row validation fails, i set e.Cancel to TRUE.
this prevents user from leaving cell/row in details grid, EXCEPT in one case :

when user inserts invalid data in details cell and directly moves focus to master grid to a record in which is not its parent, then user is not prevented from leaving that invalid cell !

further more, when i return back to THAT invalid cell, i discover that invalid value wasn't accepted, and cell has its old value.

another case: if i delete cell's value instead of inserting an invalid value, e.Cancel is also set to TRUE because that value is required. i repeat the previous scenario and when i get back to that cell, i find it EMPTY, not even having its old value...

can anyway tell me how to prevent user from leaving invalid detail cell (i.e. cancel invalid cell leaving) ?