Weird scroll bug

Hey, that is what happens when i scroll the edit box in my little program:
http://img516.imageshack.us/img516/9...geshackrb0.jpg

Here is the edit:

Code:

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                        edits[1] = CreateWindowEx(WS_EX_CLIENTEDGE,"EDIT","",ES_AUTOVSCROLL|WS_VSCROLL|WS_CHILD|WS_VISIBLE|ES_MULTILINE, 240, 50, 100, 150,
hwnd, NULL, GetModuleHandle(NULL), NULL);

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Why it happens? How do i fix it? Thanks! =]

-------EDIT--------

I actually found the problem... Its because im using

Code:

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                        SetBkMode((HDC)wParam,TRANSPARENT);

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inside WM_CTLCOLOREDIT...
How do i set the edit transparent without make that bug?

If it's a constant colour then use SetBkColor.

Well, i actually want to know why set the bkmode to transparent makes that bug and how to use it without get that error