Update window after ScrollWindow(). WinAPI - winapi

Question

I want to scroll my window, but when i do that, the previous image remains. How can i fix it?
My code, maybe there are some problems:
switch (LOWORD(wParam))
{
case SB_LINEDOWN:
iVScrollInc = 1;
break;
case SB_LINEUP:
iVScrollInc = -1;
break;
case SB_THUMBTRACK:
iVScrollInc = HIWORD(wParam) - iVScrollPos;
break;
default:
iVScrollInc = 0;
break;
}
iVScrollInc = max(-iVScrollPos, min(iVScrollInc, iVScrollMax - iVScrollPos));
if (iVScrollInc != 0)
{
iVScrollPos += iVScrollInc;
ScrollWindow(hWnd, 0, -cyChar * iVScrollInc, NULL, NULL);
SetScrollPos(hWnd, SB_VERT, iVScrollPos, TRUE);
UpdateWindow(hWnd);
}
EDIT (i cant paste all my code because of stackoverflow writes me that i have more code that describing of my question)
case WM_PAINT:{
PAINTSTRUCT ps;
HDC hDc = BeginPaint(hWnd, &ps);
for (int i = 0 ; i < buffer.size(); ++i)
TextOut(hDc, 20, 20 + i*cyChar, buffer[i].c_str(), buffer[i].length());
EndPaint(hWnd, &ps);
return 0;
}
// buffer - is std::vector<std::string>
// cyChar - is height of symbols

Answer 1

for (int i = 0 ; i < buffer.size(); ++i), it shouldn't be buffer.size().
Add int iPaintEnd = min(buffer.size(), iVscrollPos + ps.rcPaint.bottom / cyChar); Change to for (int i = 0 ; i < iPaintEnd ; ++i)
Besides, you should use std::vector<std::wstring> buffer.
Here is my code, you need refer to it.
LRESULT CALLBACK WndProc(HWND hwnd, UINT iMsg, WPARAM wParam, LPARAM lParam)
{
static int cxChar, cxCaps, cyChar, cxClient, cyClient, iMaxWidth,
iVscrollPos, iVscrollMax;
HDC hdc;
int i, y, iPaintBeg, iPaintEnd, iVscrollInc;
PAINTSTRUCT ps;
TEXTMETRIC tm;
switch (iMsg)
{
case WM_CREATE:
hdc = GetDC(hwnd);
GetTextMetrics(hdc, &tm);
cxChar = tm.tmAveCharWidth;
cxCaps = (tm.tmPitchAndFamily & 1 ? 3 : 2) * cxChar / 2;
cyChar = tm.tmHeight + tm.tmExternalLeading;
ReleaseDC(hwnd, hdc);
iMaxWidth = 40 * cxChar + 22 * cxCaps;
return 0;
case WM_SIZE:
cxClient = LOWORD(lParam);
cyClient = HIWORD(lParam);
iVscrollMax = max(0,buffer.size() + 2 - cyClient / cyChar);
iVscrollPos = min(iVscrollPos, iVscrollMax);
SetScrollRange(hwnd, SB_VERT, 0, iVscrollMax, FALSE);
SetScrollPos(hwnd, SB_VERT, iVscrollPos, TRUE);
return 0;
case WM_VSCROLL:
switch (LOWORD(wParam))
{
case SB_TOP:
iVscrollInc = -iVscrollPos;
break;
case SB_BOTTOM:
iVscrollInc = iVscrollMax - iVscrollPos;
break;
case SB_LINEUP:
iVscrollInc = -1;
break;
case SB_LINEDOWN:
iVscrollInc = 1;
break;
case SB_PAGEUP:
iVscrollInc = min(-1, -cyClient / cyChar);
break;
case SB_PAGEDOWN:
iVscrollInc = max(1, cyClient / cyChar);
break;
case SB_THUMBTRACK:
iVscrollInc = HIWORD(wParam) - iVscrollPos;
break;
default:
iVscrollInc = 0;
}
iVscrollInc = max(-iVscrollPos,
min(iVscrollInc, iVscrollMax - iVscrollPos));
if (iVscrollInc != 0)
{
iVscrollPos += iVscrollInc;
ScrollWindow(hwnd, 0, -cyChar * iVscrollInc, NULL, NULL);
SetScrollPos(hwnd, SB_VERT, iVscrollPos, TRUE);
UpdateWindow(hwnd);
}
return 0;
case WM_PAINT:
hdc = BeginPaint(hwnd, &ps);
iPaintBeg = max(0, iVscrollPos + ps.rcPaint.top / cyChar - 1);
iPaintEnd = min(buffer.size(),
iVscrollPos + ps.rcPaint.bottom / cyChar);
for (i = iPaintBeg; i < iPaintEnd; i++)
{
y = cyChar * (1 - iVscrollPos + i);
TextOut(hdc, 0, y,
buffer[i].c_str(),
buffer[i].length());
SetTextAlign(hdc, TA_LEFT | TA_TOP);
}
EndPaint(hwnd, &ps);
return 0;
case WM_DESTROY:
PostQuitMessage(0);
return 0;
}
return DefWindowProc(hwnd, iMsg, wParam, lParam);
}