ScrollWindowEx() does not scroll out text occationally.

test_scrollwindowex.c

#include <windows.h>
#include <stdio.h>
#include <string.h>

LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR szCmdLine, int iCmdShow) {
    HWND hwnd;
    MSG msg;
    WNDCLASSEX cls;

    cls.cbSize = sizeof(cls);
    cls.style = CS_HREDRAW | CS_VREDRAW;
    cls.lpfnWndProc = WndProc;
    cls.cbClsExtra = 0;
    cls.cbWndExtra = 0;
    cls.hInstance = hInstance;
    cls.hIcon = NULL;
    cls.hCursor = NULL;
    cls.hbrBackground = NULL;
    cls.lpszMenuName = NULL;
    cls.lpszClassName = "TestWindow";
    cls.hIconSm = NULL;

    RegisterClassEx(&cls);

    hwnd = CreateWindow("TestWindow", "TestWindow", WS_OVERLAPPEDWINDOW, 0, 0, 300, 300, NULL, NULL, hInstance, NULL);

    ShowWindow(hwnd, iCmdShow);
    UpdateWindow(hwnd);

    for (;;) {
        int r = GetMessage(&msg, NULL, 0, 0);
        if (r == 0 || r == -1)
            break;
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    return 0;
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam) {
    static int fontheight = 0;
    switch (msg) {
    case WM_CREATE:
        {
            HDC hdc;
            RECT rc;

            hdc = GetDC(hwnd);
            GetClientRect(hwnd, &rc);
            DrawText(hdc, "0123456789", -1, &rc, DT_CALCRECT);
            ReleaseDC(hwnd, hdc);
            fontheight = rc.bottom;
        }
        return 0;
    case WM_PAINT:
        {
            HDC hdc;
            PAINTSTRUCT ps;
            RECT rc;

            hdc = BeginPaint(hwnd, &ps);
            GetClientRect(hwnd, &rc);
            DrawText(hdc, "Press key or scroll wheel randomely.\nYou can see \"EUREKA\".", -1, &rc, 0);
            EndPaint(hwnd, &ps);
        }
        return 0;
    case WM_KEYDOWN:
    case WM_MOUSEWHEEL:
        {
            HDC hdc;

            /*
             * This text is not erased occasionally.
             */
            hdc = GetDC(hwnd);
            TextOut(hdc, 0, 0, "EUREKA", 6);
            ReleaseDC(hwnd, hdc);

            ScrollWindowEx(hwnd, 0, -fontheight, NULL, NULL, NULL, NULL, 0);
            UpdateWindow(hwnd);
        }
        return 0;
    case WM_DESTROY:
        PostQuitMessage(0);
        return 0;
    }
    return DefWindowProc(hwnd, msg, wParam, lParam);
}