Repaint failure

[cosmicvoid]

I'm using VC++ 6 on Win2k, and I am testing on XP and Vista for compatibility problems. My app has a graphic window which is not repainting properly when my app has lost focus and other windows are dragged across it. When my app is topmost, there is no problem. When a child dialog dragged across, there is no problem.

Since the repaint does occur (albiet corrupted), I know WM_PAINT is being handled. What is baffling to me is that the problem only happens in XP and Vista, not Win2K.

I am drawing to a buffer in memory and blitting to the screen:

case WM_PAINT:
	dc = BeginPaint(hWsbox, &ps);
	SelectObject(dc, hBitmap);
	BitBlt(dc, 0, 0, selarr_wid, selarr_hgt,
		sdc, 0, scroll ? si.nPos * (SEL_RY + fhgt) : 0, SRCCOPY);
	EndPaint(hWsbox, &ps);
	break;

Can someone give me a clue?