Calling QStyle::pixelMetric without style options lead to DPI scaling issues

Greetings –

Qt 5.14 introduced smarts to allow for QStyleOptions control DPI scaling. However, there is an issue where DPI scaling is wrong if this isn't provided. In QStyleHelpers, if the options aren't provided a fixed value of qstyleBaseDpi is used.

See "Widget style: Use per-screen DPI in QStyleHelper::dpiScaled()" d603ee689f0e3fdcfa3230b3d75cdce6c5af05c1 https://codereview.qt-project.org/c/qt/qtbase/+/271429 .
 

\\ Qt 5.14
Q_WIDGETS_EXPORT qreal dpi(const QStyleOption *option)
{
#ifndef Q_OS_DARWIN
    // Prioritize the application override, except for on macOS where
    // we have historically not supported the AA_Use96Dpi flag.
    if (QCoreApplication::testAttribute(Qt::AA_Use96Dpi))
        return 96;
#endif    // Expect that QStyleOption::QFontMetrics::QFont has the correct DPI set
    if (option)
        return option->fontMetrics.fontDpi();  
    return qstyleBaseDpi;
}

 
Previously the default was DPI scaled... albeit by the main monitor's logical DPI.
 

qreal dpiScaled(qreal value)
{
#ifdef Q_OS_MAC
    // On mac the DPI is always 72 so we should not scale it
    return value;
#else
    static const qreal scale = qreal(qt_defaultDpiX()) / 96.0;
    return value * scale;
#endif
}

This is a functional change that has the right idea but it really means that options must be provided everywhere for a visually consistent experience. However, this isn't the case as many places internally like QFusion aren't consistent.

// ...

// Has option
int tickSize = proxy()->pixelMetric(PM_SliderTickmarkOffset, option, widget);
switch (subControl) {
SC_SliderHandle: {
    if (slider->orientation == Qt::Horizontal) {

        // Missing option!!!
        rect.setHeight(proxy()->pixelMetric(PM_SliderThickness));
        rect.setWidth(proxy()->pixelMetric(PM_SliderLength));

// ...

As a work-around I created a proxy style that created a default style object. This only works in a single-DPI case, though.

A recommended real fix would be patching all usage points to include options and perhaps doing a better default in the no-options case (which probably should be a logged as a warning as well).

Cheers!

NOTE: I see some other bugs claiming this was fixed but I think the root issue still exists.