some thing wrong when I merge cells by row

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C#

VB

I was have function can merge cells by rows in cell_painting event : (merge cells have equals values) i.e. if we Found two same value in different cells we draw a rectangle have a size( width and height) for these 2 cells and draw string or value in the new rectangle.
but when the grid be come greater than size of form i.e. not all columns appears and i need to move scroll to show other column and the same for rows, we saw scroll bar horizontal or vertical; the problem is when i increment the scroll the rectangle drawing disappear even I write e.handled = true for rows merging.??
can any one help me please???