Broken insitialization of High DPI Scaling

I have desktop setup consisting of 2 screens and a layout like on image below

Screen scales:

#1 - 175%

#2 - 100%

and there is no any environment varialbe set that affects scaling.

Application creates QScreen object for each screen in order : #2, #1

Let's see what is happening on application start.

First, QHighDpiScaling::initHighDpiScaling() is called and it initializes static member QHighDpiScaling::m_active

// Set initial active state
m_active = m_globalScalingActive || m_usePlatformPluginDpi;

_*where _m_usePlatformPluginDpi* is true by default.

Then QScreen for display #2 is created in QWindowSystemInterface::handleScreenAdded(). Functions inside QScreen construction initilizes geometry of the screen object which are in independent device units. It's important to note that at this moment QHighDpiScaling::m_active is true and returned scale factor is correct (which is 1.0 in fact). After that QHighDpiScaling::updateHighDpiScaling() is called.

QHighDpiScaling::updateHighDpiScaling() updates value of QHighDpiScaling::m_active with code:

// Check if any screens (now) has a scale factor != 1 and set
// m_platformPluginDpiScalingActive if so.
if (m_usePlatformPluginDpi && !m_platformPluginDpiScalingActive ) {
    const auto screens = QGuiApplication::screens();
    for (QScreen *screen : screens) {
        if (!qFuzzyCompare(screenSubfactor(screen->handle()), qreal(1))) {
            m_platformPluginDpiScalingActive  = true;
            break;
        }
    }
}

m_active = m_globalScalingActive || m_screenFactorSet || m_platformPluginDpiScalingActive;

At this moment QGuiApplication::screens() contains only one screen with scale factor == 1.0. So, here is important that _m_platformPluginDpiScalingActive_ is false (2 others bool variables also false) and as result m_active becomes false too.

Next we go to creation of next QScreen object for display #1. And here initialization of screen geometry goes wrong. Geometry is calculated by function QPlatformScreen::

deviceIndependentGeometry().

QRect QPlatformScreen::deviceIndependentGeometry() const
{
    qreal scaleFactor = QHighDpiScaling::factor(this);
    QRect nativeGeometry = geometry();
    return QRect(nativeGeometry.topLeft(),   QHighDpi::fromNative(nativeGeometry.size(), scaleFactor));
}

And here qreal scaleFactor = QHighDpiScaling::factor(this); instead of scale equal to 1.75 returns 1.0. And the reason is that QHighDpiScaling::m_active is false and 1.0 is a fallback value.

Possible solution

Call QHighDpiScaling::updateHighDpiScaling() after creation of all QScreen objects and not after each one.

Temporary fix

Call QHighDpiScaling::setGlobalFactor() to set some custom scale factor and then call second one with scale == 1.0. Call of this function leads to recalculation of screens geometry.

Test project

Attachments contains test project to reproduce the bug.

main.cpp contains defs that can be enabled/disabled. Without temporary fix application outputs:

"\\\\.\\DISPLAY2" QRect(0,0 3440x1440)
"\\\\.\\DISPLAY1" QRect(-1920,0 1920x1080)

With temporary fix:

"\\\\.\\DISPLAY2" QRect(0,0 3440x1440)
"\\\\.\\DISPLAY1" QRect(-1920,0 1097x617)