关于WM_MOUSEMOVE与屏幕刷新的关联问题

我使用WM_MOUSEMOVE来动态捕获并显示当前鼠标所指像素点的RGB值——
case WM_MOUSEMOVE:
point.x = ((MOUSEHOOKSTRUCT )lParam)->pt.x;
point.y = ((MOUSEHOOKSTRUCT *)lParam)->pt.y;
/*GetCursorPos(&point);/
rectDisplay.left = pointPre.x - 10 * cxChar;
rectDisplay.top = pointPre.y - 5 * cyChar;
rectDisplay.right = pointPre.x + 20 * cxChar;
rectDisplay.bottom = pointPre.y + 5 * cyChar;
pointPre.x = point.x;
pointPre.y = point.y;

    color = GetPixel(screenHdc, point.x, point.y);
    R_G_B[0] = GetRValue(color);
    R_G_B[1] = GetGValue(color);
    R_G_B[2] = GetBValue(color);
    StringCchPrintf(szBuffer, 10, TEXT("%02X %02X %02X "), R_G_B[0], R_G_B[1], R_G_B[2]);

    SetBkColor(screenHdc, color);
    /*RedrawWindow(NULL, &rectDisplay, NULL, RDW_INVALIDATE | RDW_INTERNALPAINT | RDW_UPDATENOW | RDW_ALLCHILDREN);*/
    InvalidateRect(GetDesktopWindow(), &rectDisplay, TRUE);
    UpdateWindow(GetDesktopWindow());
    TextOut(screenHdc, point.x + cxChar, point.y + cyChar, szBuffer, 9);
    break;

测试结果是，当鼠标正在移动时，光标右下方能正确显示出文本框，但鼠标一旦停下来，文本框就消失了。
程序内的逻辑应该是鼠标移动突然停下来后，光标右下角能保留最后一次的文本框才对呀。

把你画文本框的代码TextOut等放到OnPaint中去，不然界面重绘，它就没了