Windows: rendering issues when customizing window frame with native event filters

#include "mainwindow.h"

#ifdef Q_OS_WIN
#include <Windows.h>
#include <WinUser.h>
#include <windowsx.h>
#include <Dwmapi.h>
#include <gdiplus.h>
#include <GdiPlusColor.h>
#pragma comment (lib,"Dwmapi.lib")
#endif //Q_OS_WIN

MainWindow::MainWindow(QWidget *parent)
    : QWidget(parent)
{
    setStyleSheet("background-color: rgb(83, 83, 83);");
    resize(250,200);
}

void MainWindow::showEvent(QShowEvent *event)
{
    QWidget::showEvent(event);
#ifdef Q_OS_WIN
    window_borderless();
#endif
}

#ifdef Q_OS_WIN
void MainWindow::window_borderless()
{
    if (isVisible())
    {
        window_shadow();

        SetWindowPos(reinterpret_cast<HWND>(winId()), 0, 0, 0, 0, 0, SWP_FRAMECHANGED | SWP_NOMOVE | SWP_NOSIZE);
    }
}

void MainWindow::window_shadow()
{
    const MARGINS shadow = { 1, 1, 1, 1 };
    DwmExtendFrameIntoClientArea(reinterpret_cast<HWND>(winId()), &shadow);
}

bool MainWindow::nativeEvent(const QByteArray &eventType, void *message, long *result)
{
    MSG *msg = static_cast<MSG *>(message);
    switch (msg->message)
    {
    case WM_NCCALCSIZE:
    {
        //this kills the window frame and title bar we added with
        //WS_THICKFRAME and WS_CAPTION
        *result = 0;
        return true;
        break;
    }
    case WM_NCHITTEST:
    {
        *result = 0;

        const LONG border_width = 8; //in pixels
        RECT winrect;
        GetWindowRect(reinterpret_cast<HWND>(winId()), &winrect);

        long x = GET_X_LPARAM(msg->lParam);
        long y = GET_Y_LPARAM(msg->lParam);

        bool resizeWidth = minimumWidth() != maximumWidth();
        bool resizeHeight = minimumHeight() != maximumHeight();

        if(resizeWidth)
        {
            //left border
            if (x >= winrect.left && x < winrect.left + border_width)
            {
                *result = HTLEFT;
            }
            //right border
            if (x < winrect.right && x >= winrect.right - border_width)
            {
                *result = HTRIGHT;
            }
        }
        if(resizeHeight)
        {
            //bottom border
            if (y < winrect.bottom && y >= winrect.bottom - border_width)
            {
                *result = HTBOTTOM;
            }
            //top border
            if (y >= winrect.top && y < winrect.top + border_width)
            {
                *result = HTTOP;
            }
        }
        if(resizeWidth && resizeHeight)
        {
            //bottom left corner
            if (x >= winrect.left && x < winrect.left + border_width &&
                    y < winrect.bottom && y >= winrect.bottom - border_width)
            {
                *result = HTBOTTOMLEFT;
            }
            //bottom right corner
            if (x < winrect.right && x >= winrect.right - border_width &&
                    y < winrect.bottom && y >= winrect.bottom - border_width)
            {
                *result = HTBOTTOMRIGHT;
            }
            //top left corner
            if (x >= winrect.left && x < winrect.left + border_width &&
                    y >= winrect.top && y < winrect.top + border_width)
            {
                *result = HTTOPLEFT;
            }
            //top right corner
            if (x < winrect.right && x >= winrect.right - border_width &&
                    y >= winrect.top && y < winrect.top + border_width)
            {
                *result = HTTOPRIGHT;
            }
        }

        //TODO: allow move?
        if(*result==0)
            *result = HTCAPTION ;

        return true;
        break;
    } //end case WM_NCHITTEST
    }

    return QWidget::nativeEvent(eventType, message, result);
}
#endif //Q_OS_WIN