Problem ?

I am working on tablet input and I found out that there is an odd behavior with the tablet when using the stylus button (usually simulating a mouse button right click). Basically, some TabletRelease/MouseButtonRelease seems to be missing and the information in QEvent::button() and QEvent::buttons() are inconsistent.

I made a simple application tracking tablet/mouse events, keeping a list of pressed buttons. I place the stylus on the tablet and press the stylus button and below is info about one of the events I receive:

Event: 151972
  event type: TabletPress
  watched class: QLabel
  device type: Stylus
  button: LeftButton
  buttons: RightButton
  m_keys: 
  m_buttons: LeftButton, RightButton

The problem here is that we have a TabletPress event with the button LeftButton but with buttons [RightButton], while in my private list m_buttons I have the accurate state of buttons pressed [LeftButton, RightButton].

Another problem is that later, in the same conditions as written above, I lift the stylus from the tablet: the event MouseButtonRelease/TabletRelease for the LeftButton will not be sent. The event's buttons will be correct ([RightButton]) while my private list m_buttons will not ([LeftButton, RightButton]) .

I tried to toggle the attribute Qt::AA_SynthesizeMouseForUnhandledTabletEvents but it does not change anything.

Is this the expected behavior or am I missing something ?

How to reproduce the problem

• Place the stylus on the tablet
• Press the stylus button (simulating a mouse right click button)
• Lift the stylus from the tablet (the event with inconsistent button/buttons is sent)
• Release the stylus button (there is no MouseButtonRelease/TabletRelease event sent here)

Simple test application code

I also added some fix in my code to be able to get the corresponding buttons state of the events in my private list.

mainwindow.h

#pragma once

#include <QMainWindow>
#include <QList>
#include <QObject>
#include <QEvent>
#include <QSinglePointEvent>
#include <QMetaEnum>
#include <QTextEdit>
#include <QPushButton>
#include <QLabel>

class TabletButtonInputDebugger : public QObject
{
    Q_OBJECT

public:
    TabletButtonInputDebugger(QObject* parent = nullptr);

    bool useFix() const;
    void setUseFix(bool useFix);

    void setDebugOutputTextEdit(QTextEdit* textEdit);
    void setDebugDownButtons(QLabel* label);

protected:
    bool eventFilter(QObject* watched, QEvent* event) override;

private:
    void updateInput(
        const QObject& watched,
        const QEvent& ev);

    void onKeyPress(
        const QObject& watched,
        const QKeyEvent& ev);
    void onKeyRelease(
        const QObject& watched,
        const QKeyEvent& ev);

    void onButtonPress(
        const QObject& watched,
        const QSinglePointEvent& ev);
    void onButtonRelease(
        const QObject& watched,
        const QSinglePointEvent& ev);

    void onButtonDoubleClick(
        const QObject& watched,
        const QSinglePointEvent& ev);

    void debugInput(
        const QObject& watched,
        const QInputEvent& event);

    void appendDebugLine(const QString& line);

    QList<Qt::Key> m_keys;
    QList<Qt::MouseButton> m_buttons;

    bool m_isStylusDown = false;
    bool m_isStylusDeviceUnstable = false;
    bool m_useFix = false;

    QTextEdit* m_debugOutputTextEdit = nullptr;
    QLabel* m_debugDownButtonsLabel = nullptr;
};

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);

private:
    TabletButtonInputDebugger m_tbid;
};

main_window.cpp

#include "mainwindow.h"

#include <QSplitter>
#include <QVBoxLayout>
#include <QHBoxLayout>
#include <QCheckBox>
#include <QTimer>

namespace
{

template <typename Enum>
const char* enumValueToKey(Enum val)
{
    const QMetaEnum metaEnum = QMetaEnum::fromType<Enum>();
    return metaEnum.valueToKey(val);
}

QString toString(QInputDevice::DeviceType deviceType)
{
    switch (deviceType)
    {
        default:
        case QInputDevice::DeviceType::Unknown:
            return "Unknown";
        case QInputDevice::DeviceType::Mouse: return "Mouse";
        case QInputDevice::DeviceType::TouchScreen: return "TouchScreen";
        case QInputDevice::DeviceType::TouchPad: return "TouchPad";
        case QInputDevice::DeviceType::Puck: return "Puck";
        case QInputDevice::DeviceType::Stylus: return "Stylus";
        case QInputDevice::DeviceType::Airbrush: return "Airbrush";
        case QInputDevice::DeviceType::Keyboard: return "Keyboard";
    }
}

QString toString(Qt::MouseButton button)
{
    if (button == Qt::LeftButton)
        return "LeftButton";
    if (button == Qt::RightButton)
        return "RightButton";
    if (button == Qt::MiddleButton)
        return "MiddleButton";
    if (button == Qt::BackButton)
        return "BackButton";
    if (button == Qt::ForwardButton)
        return "ForwardButton";
    if (button == Qt::TaskButton)
        return  "TaskButton";

    return "UnknownButton";
}

QString toString(Qt::MouseButtons buttons)
{
    QStringList names;

    if (buttons & Qt::LeftButton)
        names << "LeftButton";
    if (buttons & Qt::RightButton)
        names << "RightButton";
    if (buttons & Qt::MiddleButton)
        names << "MiddleButton";
    if (buttons & Qt::BackButton)
        names << "BackButton";
    if (buttons & Qt::ForwardButton)
        names << "ForwardButton";
    if (buttons & Qt::TaskButton)
        names << "TaskButton";
    if (names.isEmpty())
        names << "NoButton";

    return names.join("|");
}

QString toString(Qt::Key key)
{
    return QString(enumValueToKey(key));
}

template <typename T>
QString join(const QList<T>& list, const QString& sep)
{
    QString res;

    for (const T& e : list)
    {
        res += toString(e) + sep;
    }

    res.chop(sep.size());

    return res;
}

}

TabletButtonInputDebugger::TabletButtonInputDebugger(QObject* parent) :
    QObject(parent)
{

}

bool TabletButtonInputDebugger::useFix() const
{
    return m_useFix;
}

void TabletButtonInputDebugger::setUseFix(bool useFix)
{
    if (m_useFix == useFix)
    {
        return;
    }

    m_buttons.clear();
    m_useFix = useFix;
}

void TabletButtonInputDebugger::setDebugOutputTextEdit(QTextEdit* textEdit)
{
    m_debugOutputTextEdit = textEdit;
}

void TabletButtonInputDebugger::setDebugDownButtons(QLabel* label)
{
    m_debugDownButtonsLabel = label;
}

bool TabletButtonInputDebugger::eventFilter(QObject* watched, QEvent* event)
{
    updateInput(*watched, *event);
    return QObject::eventFilter(watched, event);
}

void TabletButtonInputDebugger::updateInput(
    const QObject& watched,
    const QEvent& ev)
{
    // Ignoring non-input events.
    if (!ev.isInputEvent())
    {
        return;
    }

    // Ignoring events on QWidgetWindow.
    if (!watched.isWidgetType())
    {
        return;
    }

    switch (ev.type())
    {
    case QEvent::KeyPress:
        onKeyPress(watched, static_cast<const QKeyEvent&>(ev));
        break;
    case QEvent::KeyRelease:
        onKeyRelease(watched, static_cast<const QKeyEvent&>(ev));
        break;
    case QEvent::MouseButtonPress:
    case QEvent::TabletPress:
        onButtonPress(watched, static_cast<const QSinglePointEvent&>(ev));
        break;
    case QEvent::MouseButtonRelease:
    case QEvent::TabletRelease:
        onButtonRelease(watched, static_cast<const QSinglePointEvent&>(ev));
        break;
    case QEvent::MouseButtonDblClick:
        onButtonDoubleClick(watched, static_cast<const QSinglePointEvent&>(ev));
        break;
    case QEvent::MouseMove:
    case QEvent::TabletMove:
        break;
    default:
        // debugInput(watched, ev);
        break;
    }
}

void TabletButtonInputDebugger::onKeyPress(
    const QObject& watched,
    const QKeyEvent& ev)
{
    if (ev.isAutoRepeat()) { return; }

    const Qt::Key key = static_cast<Qt::Key>(ev.key());
    if (!m_keys.contains(key))
    {
        m_keys.push_back(key);
    }
}

void TabletButtonInputDebugger::onKeyRelease(
    const QObject& watched,
    const QKeyEvent& ev)
{
    if (ev.isAutoRepeat()) { return; }

    m_keys.removeAll(ev.key());
}

void TabletButtonInputDebugger::onButtonPress(
    const QObject& watched,
    const QSinglePointEvent& ev)
{
    if (m_useFix)
    {
        // If the stylus is down and we receive an event from another device,
        // we cannot track its down state anymore. It is unstable.
        if (m_isStylusDown &&
            ev.deviceType() != QInputDevice::DeviceType::Stylus)
        {
            m_isStylusDown = false;
            m_isStylusDeviceUnstable = true;
            m_buttons.clear();

            appendDebugLine(" -----------------");
            appendDebugLine("| Stylus unstable |");
            appendDebugLine(" -----------------");
            appendDebugLine("");
        }
        else
        {
            if (ev.deviceType() == QInputDevice::DeviceType::Stylus)
            {
                if (m_isStylusDeviceUnstable)
                {
                    // Ignoring those events.
                    return;
                }

                if (ev.button() == Qt::LeftButton)
                {
                    m_isStylusDown = true;
                }
            }
        }
    }

    if (!m_buttons.contains(ev.button()))
    {
        m_buttons.push_back(ev.button());
    }

    debugInput(watched, ev);
}

void TabletButtonInputDebugger::onButtonRelease(
    const QObject& watched,
    const QSinglePointEvent& ev)
{
    if (m_useFix)
    {
        if (m_isStylusDeviceUnstable)
        {
            // If Qt tells that is that there are no buttons pressed,
            // the stylus is considered stable again.
            if (ev.buttons() == Qt::NoButton)
            {
                appendDebugLine(" ---------------------");
                appendDebugLine("| Stylus stable again |");
                appendDebugLine(" ---------------------");
                appendDebugLine("");

                m_isStylusDeviceUnstable = false;
                m_buttons.clear();
            }
        }
        else
        {
            if (ev.deviceType() == QInputDevice::DeviceType::Stylus &&
                ev.button() == Qt::LeftButton)
            {
                m_isStylusDown = false;
            }
        }
    }

    m_buttons.removeAll(ev.button());

    debugInput(watched, ev);
}

void TabletButtonInputDebugger::onButtonDoubleClick(
    const QObject& watched,
    const QSinglePointEvent& ev)
{
    debugInput(watched, ev);
}

void TabletButtonInputDebugger::debugInput(
    const QObject& watched,
    const QInputEvent& event)
{
    const auto eventTypeStr = enumValueToKey(event.type());
    const auto watchedClassName = watched.metaObject()->className();
    const auto deviceTypeStr = toString(event.deviceType());
    const auto m_keysStr = join(m_keys, ", ");
    const auto m_buttonsStr = join(m_buttons, ", ");

    appendDebugLine(QString() + "Event: " + QString::number(event.timestamp()));
    appendDebugLine(QString() + "  event type: " + eventTypeStr);
    appendDebugLine(QString() + "  watched class: " + watchedClassName);
    appendDebugLine(QString() + "  device type: " + deviceTypeStr);

    if (event.type() == QEvent::KeyPress ||
        event.type() == QEvent::KeyRelease)
    {
        const auto& ke = static_cast<const QKeyEvent&>(event);
        appendDebugLine(QString() + "  key: " + toString(static_cast<Qt::Key>(ke.key())));
    }
    if (event.isSinglePointEvent())
    {
        const auto& spe = static_cast<const QSinglePointEvent&>(event);
        appendDebugLine(QString() + "  button: " + toString(spe.button()));
        appendDebugLine(QString() + "  buttons: " + toString(spe.buttons()));
    }

    appendDebugLine(QString() + "  m_keys: " + m_keysStr);
    appendDebugLine(QString() + "  m_buttons: " + m_buttonsStr);

    if (m_debugDownButtonsLabel)
    {
        m_debugDownButtonsLabel->setText(m_buttonsStr);
    }

    appendDebugLine("");
}

void TabletButtonInputDebugger::appendDebugLine(const QString& line)
{
    qDebug() << line;

    if (m_debugOutputTextEdit)
    {
        m_debugOutputTextEdit->append(line);
    }
}

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent)
{
    auto centralSplitterWidget = new QSplitter(this);
    setCentralWidget(centralSplitterWidget);
    centralSplitterWidget->setHandleWidth(20);
    centralSplitterWidget->setChildrenCollapsible(false);

    auto testAreaWidget = new QLabel(centralSplitterWidget);
    centralSplitterWidget->addWidget(testAreaWidget);
    testAreaWidget->setText("Test area");
    testAreaWidget->setAlignment(Qt::AlignCenter);
    testAreaWidget->setMouseTracking(true);
    testAreaWidget->setTabletTracking(true);

    auto middlePaneWidget = new QWidget(centralSplitterWidget);
    centralSplitterWidget->addWidget(middlePaneWidget);
    auto middlePaneLayout = new QVBoxLayout(middlePaneWidget);

    auto outputTextEdit = new QTextEdit(middlePaneWidget);
    middlePaneLayout->addWidget(outputTextEdit, 1);
    outputTextEdit->setReadOnly(true);

    auto clearOutputTextEditButton = new QPushButton(middlePaneWidget);
    middlePaneLayout->addWidget(clearOutputTextEditButton);
    clearOutputTextEditButton->setText("Clear");
    connect(
        clearOutputTextEditButton, &QPushButton::clicked,
        this, [outputTextEdit]() { outputTextEdit->clear(); });

    auto rightPaneWidget = new QWidget(centralSplitterWidget);
    centralSplitterWidget->addWidget(rightPaneWidget);
    auto rightPaneLayout = new QVBoxLayout(rightPaneWidget);
    rightPaneLayout->setAlignment(Qt::AlignTop);
    rightPaneLayout->addSpacerItem(new QSpacerItem(-1, -1));

    auto downButtonsLayout = new QHBoxLayout();
    rightPaneLayout->addLayout(downButtonsLayout);

    downButtonsLayout->addWidget(new QLabel("Buttons down: ", rightPaneWidget));

    auto downButtonsLabel = new QLabel(rightPaneWidget);
    downButtonsLayout->addWidget(downButtonsLabel, 1);

    auto toggleFixCheckBox = new QCheckBox(rightPaneWidget);
    rightPaneLayout->addWidget(toggleFixCheckBox);
    toggleFixCheckBox->setText("Use fix");
    toggleFixCheckBox->setChecked(m_tbid.useFix());
    connect(
        toggleFixCheckBox, &QCheckBox::toggled,
        this, [this](bool state) { m_tbid.setUseFix(state); });

    auto synthesizeMouseForUnhandledTabletEventsButton = new QCheckBox(rightPaneWidget);
    rightPaneLayout->addWidget(synthesizeMouseForUnhandledTabletEventsButton);
    synthesizeMouseForUnhandledTabletEventsButton->setText("AA_SynthesizeMouseForUnhandledTabletEventsButton");
    synthesizeMouseForUnhandledTabletEventsButton->setChecked(qApp->testAttribute(Qt::AA_SynthesizeMouseForUnhandledTabletEvents));
    connect(
        synthesizeMouseForUnhandledTabletEventsButton, &QCheckBox::toggled,
        this, [](bool state) { qApp->setAttribute(Qt::AA_SynthesizeMouseForUnhandledTabletEvents, state); });

    // - Tablet debugger -

    testAreaWidget->installEventFilter(&m_tbid);
    // Also tested with the application, same problem.
    // qApp->installEventFilter(&m_tbid);
    m_tbid.setDebugOutputTextEdit(outputTextEdit);
    m_tbid.setDebugDownButtons(downButtonsLabel);
}

main.cpp

#include "mainwindow.h"

#include <QApplication>

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

    MainWindow w;
    w.show();

    return app.exec();
}

Info

OS: Windows 11
Qt: 6.9.1 (I also tested with Qt 6.7.2.)