A flexible QScrollBar that can take values outside of its original range by automatically extending its range

FlexibleScrollBar.cpp

class FlexibleScrollBar : public QScrollBar
{
    Q_OBJECT

public:
    FlexibleScrollBar(Qt::Orientation orientation, QWidget *parent = nullptr)
        : QScrollBar(orientation, parent)
        , mOverrideMinimum(0)
        , mOverrideMaximum(0)
        , mDesiredMinimum(0)
        , mDesiredMaximum(0)
        , mChangingRange(false)
    {}

    void forceSetValue(int value)
    {
        if (value < minimum())
            setOverrideMinimum(value);
        if (value > maximum())
            setOverrideMaximum(value);

        setValue(value);
    }

protected:
    void sliderChange(SliderChange change) override
    {
        switch (change) {
        case SliderRangeChange:
            if (!mChangingRange) {
                mDesiredMinimum = minimum();
                mDesiredMaximum = maximum();

                // shrink only as much as allowed by the current value
                if (minimum() > value() || maximum() < value()) {
                    mChangingRange = true;
                    setRange(std::min(mDesiredMinimum, value()),
                             std::max(mDesiredMaximum, value()));
                    mChangingRange = false;
                    return;
                }
            }
            break;
        case SliderValueChange:
            // shrink range back to desired range as much as possible
            if (mOverrideMinimum < mDesiredMinimum)
                setOverrideMinimum(std::min(mDesiredMinimum, value()));
            if (mOverrideMaximum > mDesiredMaximum)
                setOverrideMaximum(std::max(mDesiredMaximum, value()));
            break;
        case SliderOrientationChange:
        case SliderStepsChange:
            break;
        }

        QScrollBar::sliderChange(change);
    }

private:
    void setOverrideMinimum(int min)
    {
        mOverrideMinimum = min;
        mChangingRange = true;
        setMinimum(min);
        mChangingRange = false;
    }

    void setOverrideMaximum(int max)
    {
        mOverrideMaximum = max;
        mChangingRange = true;
        setMaximum(max);
        mChangingRange = false;
    }

    int mOverrideMinimum;
    int mOverrideMaximum;
    int mDesiredMinimum;
    int mDesiredMaximum;
    bool mChangingRange;
};