How can I customize the title of a QGroupBox using a QProxyStyle?

[l3u_]

Hi all,

I want to (slightly) customize the title of QGroupBoxes, but without using style sheets. Mostly to work around the problems with QSpinBoxes that emerged with Qt 6.8.2 (cf. Bug #133845).

Until now, I set the following application-wide style sheet:

QGroupBox[checkable=false] { font-weight: bold; }

and for those I didn't want the title to be bold, I set (per group box):

QGroupBox { font-weight: normal; }

That did the trick. However, I now try to get the same result using a QProxyStyle. I added a property defaultTitle to mark those boxes who should get a non-bold title. This seemed to work fine by simply tweaking the style option and painter font. here's a minimal example:

#include <QApplication>
#include <QWidget>
#include <QVBoxLayout>
#include <QGroupBox>
#include <QLabel>
#include <QProxyStyle>
#include <QStyleOptionGroupBox>
#include <QPainter>
#include <QDebug>

class Demo : public QWidget
{
public:
    Demo()
    {
        auto *layout = new QVBoxLayout(this);
        QGroupBox *box;
        QVBoxLayout *boxLayout;

        box = new QGroupBox(tr("Custom title"));
        boxLayout = new QVBoxLayout(box);
        boxLayout->addWidget(new QLabel(tr("Some content")));
        layout->addWidget(box);

        box = new QGroupBox(tr("Checkable box"));
        box->setCheckable(true);
        boxLayout = new QVBoxLayout(box);
        boxLayout->addWidget(new QLabel(tr("Some content")));
        layout->addWidget(box);

        box = new QGroupBox(tr("Default title"));
        box->setProperty("defaultTitle", true);
        boxLayout = new QVBoxLayout(box);
        boxLayout->addWidget(new QLabel(tr("Some content")));
        layout->addWidget(box);

        box = new QGroupBox;
        boxLayout = new QVBoxLayout(box);
        boxLayout->addWidget(new QLabel(tr("Some content")));
        layout->addWidget(box);
    }
};

class Style : public QProxyStyle
{
public:
    void drawComplexControl(ComplexControl control, const QStyleOptionComplex *option,
                            QPainter *painter, const QWidget *widget) const override
    {
        if (control == CC_GroupBox) {
            if (const auto *groupBoxOption = qstyleoption_cast<const QStyleOptionGroupBox *>(option)) {
                if (! groupBoxOption->text.isEmpty()
                    && ! (groupBoxOption->subControls & QStyle::SC_GroupBoxCheckBox)
                    && ! widget->property("defaultTitle").toBool()) {

                    // Copy the option
                    QStyleOptionGroupBox opt = *groupBoxOption;

                    // Set a bold font
                    QFont font;
                    font.setBold(true);
                    QFontMetrics fm(font);
                    opt.fontMetrics = fm;
                    painter->setFont(font);

                    // Let the base style draw the box
                    QProxyStyle::drawComplexControl(control, &opt, painter, widget);
                    return;
                }
            }
        }

        QProxyStyle::drawComplexControl(control, option, painter, widget);
    }
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    app.setStyle(new Style);
    Demo demo;
    demo.show();
    return app.exec();
}

This works using the Fusion style. However, it does not affect e.g. Breeze or the macOS style. I looked into Breeze, and it simply overwrites my font and font metrics settings and thus draws the default stuff. I suppose the macOS style does the same.

I messed with this quite long. However, it seems that replacing this really minimalist style sheet is not that easy. So here's my question: Is there a straightforward way to make QGroupBox titles bold using a QProxyStyle? Or do I really have to draw the whole thing myself (calculating the rects and so on) to make this relibaly work with different base styles (Fusion, Breeze, Windows, macOS)?

Did I miss something? Thanks for all help!