off-screen drawing and BitBlt onto drawgrid.canvas problem

I have problems with drawing: I use off-screen bitmap to draw Grid
image then I BitBlit it onto DrawGrid.Canvas. When the grid is scrolled
down I get first row painted on the grid, though it IS unvisible. I
tried to save to file before I BitBlt(..., SRCCOPY) image onto the grid
canvas. Image was OK, so the problem lies somewhere in the VCL. Why do
I get garbage painted? Any ideas?

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