main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQuickWindow>
#include <Windows.h>
#include <windowsx.h>
#include "dwmapi.h"
#include "core/window_manager/WindowManager.h"

WNDPROC prevWndProc;

LRESULT CALLBACK WndProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    switch (uMsg)
    {
        case WM_NCCALCSIZE: {
            return 0;
        }
        case WM_NCHITTEST: {
            const LONG borderWidth = 5; //in pixels
            RECT winrect;
            GetWindowRect(hwnd, &winrect);
            long x = GET_X_LPARAM(lParam);
            long y = GET_Y_LPARAM(lParam);

            //bottom left corner
            if (x >= winrect.left && x < winrect.left + borderWidth &&
                 y < winrect.bottom && y >= winrect.bottom - borderWidth)
            {
                return HTBOTTOMLEFT;
            }
            //bottom right corner
            if (x < winrect.right && x >= winrect.right - borderWidth &&
                 y < winrect.bottom && y >= winrect.bottom - borderWidth)
            {
                return HTBOTTOMRIGHT;
            }
            //top left corner
            if (x >= winrect.left && x < winrect.left + borderWidth &&
                 y >= winrect.top && y < winrect.top + borderWidth)
            {
                return HTTOPLEFT;
            }
            //top right corner
            if (x < winrect.right && x >= winrect.right - borderWidth &&
                 y >= winrect.top && y < winrect.top + borderWidth)
            {
                return HTTOPRIGHT;
            }
            //left border
            if (x >= winrect.left && x < winrect.left + borderWidth)
            {
                return HTLEFT;
            }
            //right border
            if (x < winrect.right && x >= winrect.right - borderWidth)
            {
                return HTRIGHT;
            }
            //bottom border
            if (y < winrect.bottom && y >= winrect.bottom - borderWidth)
            {
                return HTBOTTOM;
            }
            //top border
            if (y >= winrect.top && y < winrect.top + borderWidth)
            {
                return HTTOP;
            }
            if(y - winrect.top > 0 && y - winrect.top < 40) {
                return HTCAPTION;
            }
        }

        return CallWindowProc(prevWndProc, hwnd, uMsg, wParam, lParam);
    }

    return CallWindowProc(prevWndProc, hwnd, uMsg, wParam, lParam);
}

int main(int argc, char *argv[])
{
#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);
    engine.load(url);

    if(QWindow* window = qobject_cast<QWindow*>( engine.rootObjects().at(0))) {
        HWND hwnd = (HWND) window->winId();
        prevWndProc = (WNDPROC) SetWindowLongPtr(hwnd, GWLP_WNDPROC, (LONG_PTR)WndProc);

        MARGINS margins = { 5, 5, 5, 5 };
        DwmExtendFrameIntoClientArea((HWND) window->winId(), &margins);
        SetWindowPos(hwnd, NULL, 0,0,0,0, SWP_FRAMECHANGED | SWP_NOMOVE | SWP_NOSIZE | SWP_NOZORDER | SWP_NOOWNERZORDER);
    }

    return app.exec();
}

main.qml

import QtQuick 2.12
import QtQuick.Window 2.12
import com.july.manager 1.0

Window {
    id: mainWindow
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")
    color: "#212121"

    Rectangle {
        width: parent.width
        height: 40
        color: "#3d3d3d"
    }

    Text {
        anchors.centerIn: parent
        text: "Text"
        font.pixelSize: 30
        color: "white"
    }
}