QScreen::availableGeometry().topLeft() is wrong for high-DPI displays

QScreen::geometry() has this thing where there size of the QRect is in device-independent coordinates and the topLeft is in device coordinates. In some cases QScreen::availableGeometry().topLeft() will be computed by mixing device-independent and device-dependent coordinates leading to a nonsense value. The case where this happens is when availableGeometry.topLeft() == geometry.topLeft().

The relevant code in src\gui\kernel\qscreen.cpp looks like this:

void QScreenPrivate::updateGeometry()
{
    qreal scaleFactor = QHighDpiScaling::factor(platformScreen);
    QRect nativeGeometry = platformScreen->geometry();
    geometry = QRect(nativeGeometry.topLeft(), QHighDpi::fromNative(nativeGeometry.size(), scaleFactor));
    availableGeometry = QHighDpi::fromNative(platformScreen->availableGeometry(), scaleFactor, geometry.topLeft());

The line for geometry explicitly preserves topLeft and only scales the size, whereas availableGeometry is handled differently. Here the topleft will be computed via these functions from src\gui\kernel\qhighdpiscaling_p.h:

inline QPoint scale(const QPoint &pos, qreal scaleFactor, QPoint origin = QPoint(0, 0))
{
     return (pos - origin) * scaleFactor + origin;
}

inline QRect scale(const QRect &rect, qreal scaleFactor, QPoint origin = QPoint(0, 0))
{
    return QRect(scale(rect.topLeft(), scaleFactor, origin), scale(rect.size(), scaleFactor));
}

template <typename T>
inline T fromNative(const T &value, qreal scaleFactor, QPoint origin = QPoint(0, 0))
{
    return scale(value, qreal(1) / scaleFactor, origin);
}

I believe the fix is compute availableGeometry in exactly the same way as geometry.

Background info

The behaviour for screen size and screen position is documented in https://doc.qt.io/qt-6/highdpi.html where it says:

Specifically, Qt will scale the screen size (resulting in a "smaller" screen for positive scale factors), but will not change the screen position. This will produce an islands-of-screens type virtual desktop geometry.