FlowLayout doesn't take care of actual content size

[Mark81]

Here my "fork" of FlowLayout, which can place the items also in vertical order:

flowlayout.h

#ifndef FLOWLAYOUT_H
#define FLOWLAYOUT_H

#include <QLayout>
#include <QRect>
#include <QStyle>

class FlowLayout : public QLayout
{
public:
    explicit FlowLayout(QWidget *parent, int margin = -1, int hSpacing = -1, int vSpacing = -1);
    explicit FlowLayout(int margin = -1, int hSpacing = -1, int vSpacing = -1);
    ~FlowLayout() override;

    void addItem(QLayoutItem *item) override;
    int horizontalSpacing() const;
    int verticalSpacing() const;
    Qt::Orientations expandingDirections() const override;
    void setOrientation(Qt::Orientations orientation) { _orientation = orientation; }
    bool hasHeightForWidth() const override;
    int heightForWidth(int) const override;
    int count() const override;
    QLayoutItem *itemAt(int index) const override;
    QSize minimumSize() const override;
    void setGeometry(const QRect &rect) override;
    QSize sizeHint() const override;
    QLayoutItem *takeAt(int index) override;

private:
    int doLayout(const QRect &rect, bool testOnly) const;
    int smartSpacing(QStyle::PixelMetric pm) const;

    QList<QLayoutItem *> _itemList;
    Qt::Orientations _orientation;
    int _hSpace;
    int _vSpace;
};

#endif // FLOWLAYOUT_H

flowlayout.cpp

#include <QtWidgets>
#include "flowlayout.h"

FlowLayout::FlowLayout(QWidget *parent, int margin, int hSpacing, int vSpacing) : QLayout(parent), _hSpace(hSpacing), _vSpace(vSpacing)
{
    setContentsMargins(margin, margin, margin, margin);
    _orientation = Qt::Horizontal;
}

FlowLayout::FlowLayout(int margin, int hSpacing, int vSpacing) : _hSpace(hSpacing), _vSpace(vSpacing)
{
    setContentsMargins(margin, margin, margin, margin);
    _orientation = Qt::Horizontal;
}

FlowLayout::~FlowLayout()
{
    QLayoutItem *item;
    while ((item = takeAt(0))) delete item;
}

void FlowLayout::addItem(QLayoutItem *item)
{
    _itemList.append(item);
}

int FlowLayout::horizontalSpacing() const
{
    if (_hSpace >= 0) return _hSpace;
    else return smartSpacing(QStyle::PM_LayoutHorizontalSpacing);
}

int FlowLayout::verticalSpacing() const
{
    if (_vSpace >= 0) return _vSpace;
    else return smartSpacing(QStyle::PM_LayoutVerticalSpacing);
}

int FlowLayout::count() const
{
    return _itemList.size();
}

QLayoutItem *FlowLayout::itemAt(int index) const
{
    return _itemList.value(index);
}

QLayoutItem *FlowLayout::takeAt(int index)
{
    if (index >= 0 && index < _itemList.size()) return _itemList.takeAt(index);
    else return nullptr;
}

Qt::Orientations FlowLayout::expandingDirections() const
{
    return nullptr;
}

bool FlowLayout::hasHeightForWidth() const
{
    return false;
}

int FlowLayout::heightForWidth(int width) const
{
    int height = doLayout(QRect(0, 0, width, 0), true);
    return height;
}

void FlowLayout::setGeometry(const QRect &rect)
{
    QLayout::setGeometry(rect);
    doLayout(rect, false);
}

QSize FlowLayout::sizeHint() const
{
    return minimumSize();
}

QSize FlowLayout::minimumSize() const
{
    QSize size;
    QLayoutItem *item;
    foreach (item, _itemList) size = size.expandedTo(item->minimumSize());
    size += QSize(2 * margin(), 2 * margin());
    return size;
}

int FlowLayout::doLayout(const QRect &rect, bool testOnly) const
{
    int left, top, right, bottom;
    getContentsMargins(&left, &top, &right, &bottom);
    QRect effectiveRect = rect.adjusted(+left, +top, -right, -bottom);
    int x = effectiveRect.x();
    int y = effectiveRect.y();
    int lineHeight = 0;
    int colWidth = 0;

    QLayoutItem *item;
    foreach (item, _itemList)
    {
        QWidget *widget = item->widget();
        int spaceX = horizontalSpacing();
        if (spaceX == -1) spaceX = widget->style()->layoutSpacing(QSizePolicy::PushButton, QSizePolicy::PushButton, Qt::Horizontal);
        int spaceY = verticalSpacing();
        if (spaceY == -1) spaceY = widget->style()->layoutSpacing(QSizePolicy::PushButton, QSizePolicy::PushButton, Qt::Vertical);

        switch (_orientation) {
        case Qt::Horizontal: {
            int nextX = x + item->sizeHint().width() + spaceX;
            if (nextX - spaceX > effectiveRect.right() && lineHeight > 0)
            {
                x = effectiveRect.x();
                y = y + lineHeight + spaceY;
                nextX = x + item->sizeHint().width() + spaceX;
                lineHeight = 0;
            }
            if (!testOnly) item->setGeometry(QRect(QPoint(x, y), item->sizeHint()));
            x = nextX;
            lineHeight = qMax(lineHeight, item->sizeHint().height());
            break; }

        case Qt::Vertical: {
            int nextY = y + item->sizeHint().height() + spaceY;
            if (nextY - spaceY > effectiveRect.bottom() && colWidth > 0)
            {
                x = x + colWidth + spaceX;
                y = effectiveRect.y();
                nextY = y + item->sizeHint().height() + spaceY;
                colWidth = 0;
            }
            if (!testOnly) item->setGeometry(QRect(QPoint(x, y), item->sizeHint()));
            y = nextY;
            colWidth = qMax(colWidth, item->sizeHint().width());
            break; }
        }

    }

    if (_orientation == Qt::Horizontal) return y + lineHeight - rect.y() + bottom;
    else return x + colWidth - rect.x() + right;
}

int FlowLayout::smartSpacing(QStyle::PixelMetric pm) const
{
    QObject *parent = this->parent();
    if (!parent)
    {
        return -1;
    }
    else if (parent->isWidgetType())
    {
        QWidget *pw = static_cast<QWidget *>(parent);
        return pw->style()->pixelMetric(pm, nullptr, pw);
    }
    else
    {
        return static_cast<QLayout *>(parent)->spacing();
    }
}

I'm testing it mainly with vertical orientation. The problem is the horizontal scroll bar appears only when the width is less than a specific value (about 480 px) regardless the actual contents.

I think it's related to the sizeHint() function. I don't understand how it can calculate the needed space in both direction without doing a layout(). Well, in fact it doesn't work!

What should I change to make the horizontal scrollbar behavior reliable like the vertical one?