I’m working on a Qt application where I have an overlay window that needs to maintain a square aspect ratio regardless of how it is resized. My app has two windows open at the same time and both are top-level windows. I want the overlay window which is a QFrame to stay as a square when resized from corner.

This is the code for my overlay window class:

#include "overlaywindow.h"

OverlayWindow::OverlayWindow(QFrame* parent) :
    QFrame(parent),
    m_isMeasurementRunning(false),
    m_aspectRatio(1.0)
{
    // Set initial appearance, and behavior of the overlay window.
    setWindowOpacity(0.5);
    setStyleSheet("border: 1.5px solid black;background-color:#A9A9A9;");
    setCursor(Qt::SizeAllCursor);
    setMinimumSize(50, 50);

    // Set maximum size based on the screen dimensions and position the window.
    QRect screenRect = screen()->geometry();
    setMaximumSize(screenRect.width(), screenRect.height());
    setGeometry(screenRect.width() / 2 - 125, screenRect.height() / 2 - 125, 250, 250);
}

void OverlayWindow::setMeasurementState(bool state)
{
    m_isMeasurementRunning = state;

    // Adjust window properties based on the measurement state.
    if (state)
    {
        setFixedSize(geometry().width(), geometry().width());
        setCursor(Qt::ForbiddenCursor);
    }
    else
    {
        QRect screenRect = screen()->geometry();
        setMaximumSize(screenRect.width(), screenRect.height());
        setMinimumSize(50, 50);
        setCursor(Qt::SizeAllCursor);
    }
}

void OverlayWindow::mousePressEvent(QMouseEvent* event)
{
    m_dragPos = event->pos();
}

void OverlayWindow::mouseMoveEvent(QMouseEvent* event)
{
    if (m_isMeasurementRunning)
    {
        // Ignore mouse move events if measurement is running.
        event->ignore();
    }
    else
    {
        // Move the window while dragging, ensuring it stays within screen bounds.
        if (checkBounds(event))
        {
            move(
                event->globalPosition().x() - m_dragPos.x(),
                event->globalPosition().y() - m_dragPos.y());
        }
    }
}

void OverlayWindow::resizeEvent(QResizeEvent* event)
{
    // Maintain aspect ratio by making the window square during resize.
    if (event->size().width() != event->oldSize().width())
    {
        resize(event->size().height(), event->size().height());
    }
    else
    {
        resize(event->size().width(), event->size().width());
    }
}

And this is the controller:

#include "windowscontroller.h"

WindowsController::WindowsController(
    MainWindow*    mainWindow,
    OverlayWindow* overlayWindow,
    FrameCounter*  framecounter,
    QString        logDirectory) :
    m_mainWindow(mainWindow),
    m_overlayWindow(overlayWindow),
    m_fpslogger(logDirectory),
    m_framecounter(framecounter),
    m_exit(false),
    m_commandLineExec(false)
{
    // Connect signals from MainWindow to controller's slots
    connect(
        m_mainWindow,
        &MainWindow::measurementToggled,
        this,
        &WindowsController::onToggleMeasurement);
    connect(m_mainWindow, &MainWindow::exitRequested, this, &WindowsController::onExitRequested);

    // Connect signals from OverlayWindow to controller's slots
    connect(
        m_overlayWindow,
        &OverlayWindow::exitRequested,
        this,
        &WindowsController::onExitRequested);

    // Connect signals from FrameCounter to controller's slots
    connect(m_framecounter, &FrameCounter::newfpsvalue, this, &WindowsController::updateDisplay);
}

void WindowsController::showWindows()
{
    // Display the main window and the overlay window
    m_mainWindow->setWindowFlags(Qt::Window);

    m_overlayWindow->setWindowFlags(
        Qt::Window | Qt::CustomizeWindowHint | Qt::WindowStaysOnTopHint);

    m_overlayWindow->show();
    m_mainWindow->show();
}

The current implementation uses the resizeEvent function to maintain the aspect ratio of the window as a square by setting the width and height to be equal. But when I try to resize from top or left the overlay window moves as well. And from what I understood heightForWidth() doesn’t work for top-level windows. I also tried multiple solution found in the forum like this one or this one but none seem to be working.

What modifications should I make to ensure that the overlay window always maintains a square aspect ratio, even when resized by the user?

Thank you!