The cursor behavior is inconsistent after selecting an item in QComboBox using different methods

Question

I customized a dialog that contains a QComboBox control m_box and a QListWidget control m_lst. The m_box supports search matching. If an item from the dropdown list of m_box is selected by directly clicking with the mouse, the cursor will not be displayed in m_box, as shown in the example below.

If an item from m_box is selected using the search matching method with the up and down keys and the Enter key, the cursor will also not be displayed in m_box, as shown in the example below.

However, if matching items are displayed through search matching and an item from the dropdown list is selected with the mouse, the cursor will then be displayed in m_box, as shown in the example below.

I hope for consistent behavior across all three methods: as long as the selected item exists in the candidate items of m_box, the cursor should not be displayed in m_box, unless the mouse is clicked on m_box, at which point the cursor should appear. Here’s a minimal reproducible example:

#include <QApplication>
#include <QDialog>
#include <QComboBox>
#include <QListWidget>
#include <QCompleter>
#include <QStringList>
#include <QVBoxLayout>
#include <QDebug>

class CustomDialog : public QDialog
{
    Q_OBJECT
public:
    CustomDialog(QWidget* parent = nullptr) : QDialog(parent)
    {
        QStringList items;
        items << "item1" << "item2";
        m_box = new QComboBox(this);
        m_box->setEditable(true);
        m_box->addItems(items);
        QCompleter* completer = new QCompleter(items, m_box);
        completer->setFilterMode(Qt::MatchContains);
        m_box->setCompleter(completer);

        m_lst = new QListWidget(this);
        m_lst->addItems(items);

        QVBoxLayout* layout = new QVBoxLayout(this);
        layout->addWidget(m_box);
        layout->addWidget(m_lst);

        connect(m_box, SIGNAL(activated(int)), this, SLOT(onSelChangedBox(int)));
    }

public slots:
    void onSelChangedBox(int iSel);

private:
    QComboBox* m_box;
    QListWidget* m_lst;
};

void CustomDialog::onSelChangedBox(int iSel)
{
    qDebug() << "clear focus of m_box";
    m_lst->setFocus();
}

int main(int argc, char* argv[])
{
    QApplication a(argc, argv);

    CustomDialog dlg;
    dlg.setFixedSize(300, 200);
    dlg.show();

    return a.exec();
}

#include "main.moc"

Note: If a second QComboBox is added and the item is selected using the third method, then clicking on the second QComboBox will result in two cursors appearing. In fact, at this point, the focus is no longer on the first QComboBox, but the cursor is still displayed. The code and example are as follows:

#include <QApplication>
#include <QDialog>
#include <QComboBox>
#include <QListWidget>
#include <QCompleter>
#include <QStringList>
#include <QVBoxLayout>
#include <QDebug>

class CustomDialog : public QDialog
{
    Q_OBJECT
public:
    CustomDialog(QWidget* parent = nullptr) : QDialog(parent)
    {
        QStringList items;
        items << "item1" << "item2";
        m_box1 = new QComboBox(this);
        m_box1->setEditable(true);
        m_box1->addItems(items);
        QCompleter* completer1 = new QCompleter(items, m_box1);
        completer1->setFilterMode(Qt::MatchContains);
        m_box1->setCompleter(completer1);

        m_box2 = new QComboBox(this);
        m_box2->setEditable(true);
        m_box2->addItems(items);
        QCompleter* completer2 = new QCompleter(items, m_box2);
        completer2->setFilterMode(Qt::MatchContains);
        m_box2->setCompleter(completer2);

        m_lst = new QListWidget(this);
        m_lst->addItems(items);

        QVBoxLayout* layout = new QVBoxLayout(this);
        layout->addWidget(m_box1);
        layout->addWidget(m_box2);
        layout->addWidget(m_lst);

        connect(m_box1, SIGNAL(activated(int)), this, SLOT(onSelChangedBox(int)));
        connect(m_box2, SIGNAL(activated(int)), this, SLOT(onSelChangedBox(int)));
    }

public slots:
    void onSelChangedBox(int iSel);

private:
    QComboBox* m_box1;
    QComboBox* m_box2;
    QListWidget* m_lst;
};

void CustomDialog::onSelChangedBox(int iSel)
{
    qDebug() << "clear focus of m_box";
    m_lst->setFocus();
}

int main(int argc, char* argv[])
{
    QApplication a(argc, argv);

    CustomDialog dlg;
    dlg.setFixedSize(500, 500);
    dlg.show();

    return a.exec();
}

#include "main.moc"

Answer 1

This seems to be a Qt bug. It may be a good idea to report this issue using the official Qt bug reporting system.

The problem is that the focus out event is eaten by the popup QCompleter.

Easiest work around is to allow Qt to close the popup normally by delaying the focus change (as already suggested by @musicamante in his comments):

void CustomDialog::onSelChangedBox(int iSel)
{
    qDebug() << "clear focus of m_box";
    QTimer::singleShot(0, this, [this](){m_lst->setFocus();});
}

Another work around is to use QCompleter::InlineCompletion instead of the popup completion.

Answer 2

I've changed my answer to show an additional observation after playing around with your example. With the following connections, you get the desired behaviour for both mouse clicks and key presses:

class CustomDialog : public QDialog
{
    Q_OBJECT
public:
    CustomDialog(QWidget* parent = nullptr) : QDialog(parent)
    {
        QStringList items;
        items << "item1" << "item2";
        m_box = new QComboBox( this);
        m_box->setEditable(true);
        m_box->addItems(items);
        QCompleter* completer = new QCompleter(items, m_box);
        completer->setFilterMode(Qt::MatchContains);
        m_box->setCompleter(completer);

        m_lst = new QListWidget(this);
        m_lst->addItems(items);

        QVBoxLayout* layout = new QVBoxLayout(this);
        layout->addWidget(m_box);
        layout->addWidget(m_lst);

        connect(completer->popup(), &QAbstractItemView::activated, this, &CustomDialog::onSelChangedBox);
        connect(completer, QOverload<QModelIndex const&>::of(&QCompleter::activated), this, &CustomDialog::onSelChangedBox);
    }

public slots:
    void onSelChangedBox(QModelIndex const&);

private:
    QComboBox* m_box;
    QListWidget* m_lst;
};

void CustomDialog::onSelChangedBox(QModelIndex const& index)
{
    qDebug() << "clear focus of m_box";
    m_lst->setFocus();
    m_lst->setCurrentRow(index.row());
}

Note that the order of the connections matters in this case and the slot function is called twice for mouse clicks. Also, adding a connection to QComboBox::activated unfortunately reverts back to the behaviour you described.

Since the full list of items is already shown in the box layout, you could consider replacing the combobox with a simple line edit.