Force aspect ratio on window

[Grand_Titan]

I have a second dialog style window which is started in mainwindow.cpp like this:
externalDisplay = new ExternalDisplay(this);
externalDisplay->show();
I want to allow resizing that window but the aspect ratio must not change. I tried using event filters like this:

ExternalDisplay::ExternalDisplay(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::ExternalDisplay)
{
    ui->setupUi(this);

    this->installEventFilter(this);
}

bool ExternalDisplay::eventFilter(QObject* obj, QEvent* event) {
    if (event->type() == QEvent::Resize) {
        QResizeEvent* resizeEvent = static_cast<QResizeEvent*>(event);
        qDebug() << "Window resized to: " << resizeEvent->size();

        int newWidth = resizeEvent->size().width();
        int newHeight = (5 * newWidth) / 7;

        this->resize(newWidth, newHeight);
    }
    return false;
}

I want the aspect ratio to be 7/5 but with this solution the window is flickering because it keeps changing between the users input and the forced aspect ratio. Is there a better solution?

[SamiV123]

@Grand_Titan

all I think off is actually not resize the window while the user is resizing it but visualize it only and only resize the window/dialog once after the resize operation stops.

[Grand_Titan]

@SamiV123 Is there no built in feature for this?