Introduce QEvent::isInputEvent

This makes it easier to reliably maintain input-event related states
in widgets, in particluar the state of keyboard modifiers. Instead of
testing for all possible event types, code can just test the flag before
safely static_cast'ing to QInputEvent to access the modifiers.

Simplify the code in QAbstractItemView accordingly.

Task-number: QTBUG-73829
Change-Id: Idc7c08e2f3f1e8844f5c6693c195153038ee6490
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/corelib/kernel/qcoreevent.cpp

@@ -295,7 +295,7 @@ QT_BEGIN_NAMESPACE
     Contructs an event object of type \a type.
 */
 QEvent::QEvent(Type type)
-    : d(nullptr), t(type), posted(false), spont(false), m_accept(true)
+    : d(nullptr), t(type), posted(false), spont(false), m_accept(true), m_inputEvent(false)
 {
     Q_TRACE(QEvent_ctor, this, t);
 }
@@ -317,6 +317,14 @@ QEvent::QEvent(const QEvent &other)
     Q_ASSERT_X(!d, "QEvent", "Impossible, this can't happen: QEventPrivate isn't defined anywhere");
 }
 
+/*!
+    \internal
+    \since 6.0
+    \fn QEvent::QEvent(Type type, QEvent::InputEventTag)
+
+    Constructs an event object of type \a type, setting the inputEvent flag to true.
+*/
+
 /*!
     \internal
     Attempts to copy the \a other event.
@@ -407,6 +415,14 @@ QEvent::~QEvent()
     The return value of this function is not defined for paint events.
 */
 
+/*!
+    \fn bool QEvent::isInputEvent() const
+    \since 6.0
+
+    Returns \c true if the event object is a QInputEvent or one of its
+    subclasses.
+*/
+
 namespace {
 template <size_t N>
 struct QBasicAtomicBitField {

src/corelib/kernel/qcoreevent.h

@@ -307,9 +307,13 @@ public:
     inline void accept() { m_accept = true; }
     inline void ignore() { m_accept = false; }
 
+    inline bool isInputEvent() const noexcept { return m_inputEvent; }
+
     static int registerEventType(int hint = -1) noexcept;
 
 protected:
+    struct InputEventTag { explicit InputEventTag() = default; };
+    QEvent(Type type, InputEventTag) : QEvent(type) { m_inputEvent = true; }
     QEventPrivate *d;
     ushort t;
 
@@ -317,7 +321,8 @@ private:
     ushort posted : 1;
     ushort spont : 1;
     ushort m_accept : 1;
-    ushort reserved : 13;
+    ushort m_inputEvent : 1;
+    ushort reserved : 12;
 
     friend class QCoreApplication;
     friend class QCoreApplicationPrivate;

src/gui/kernel/qevent.cpp

@@ -163,7 +163,7 @@ QEnterEvent::~QEnterEvent()
   \internal
 */
 QInputEvent::QInputEvent(Type type, const QInputDevice *dev, Qt::KeyboardModifiers modifiers)
-    : QEvent(type), m_dev(dev), m_modState(modifiers)
+    : QEvent(type, QEvent::InputEventTag{}), m_dev(dev), m_modState(modifiers)
 {}
 
 /*!

src/widgets/itemviews/qabstractitemview.cpp

@@ -3917,21 +3917,9 @@ QItemSelectionModel::SelectionFlags QAbstractItemView::selectionCommand(const QM
                                                                         const QEvent *event) const
 {
     Q_D(const QAbstractItemView);
-    Qt::KeyboardModifiers keyModifiers = Qt::NoModifier;
-    if (event) {
-        switch (event->type()) {
-            case QEvent::MouseButtonDblClick:
-            case QEvent::MouseButtonPress:
-            case QEvent::MouseButtonRelease:
-            case QEvent::MouseMove:
-            case QEvent::KeyPress:
-            case QEvent::KeyRelease:
-                keyModifiers = (static_cast<const QInputEvent*>(event))->modifiers();
-                break;
-            default:
-                break;
-        }
-    }
+    Qt::KeyboardModifiers keyModifiers = event && event->isInputEvent()
+                                       ? static_cast<const QInputEvent*>(event)->modifiers()
+                                       : Qt::NoModifier;
     switch (d->selectionMode) {
         case NoSelection: // Never update selection model
             return QItemSelectionModel::NoUpdate;
@@ -3989,16 +3977,16 @@ QItemSelectionModel::SelectionFlags QAbstractItemViewPrivate::extendedSelectionC
 {
     Qt::KeyboardModifiers modifiers = Qt::NoModifier;
     if (event) {
+        if (event->isInputEvent())
+            modifiers = static_cast<const QInputEvent*>(event)->modifiers();
         switch (event->type()) {
         case QEvent::MouseMove: {
             // Toggle on MouseMove
-            modifiers = static_cast<const QMouseEvent*>(event)->modifiers();
             if (modifiers & Qt::ControlModifier)
                 return QItemSelectionModel::ToggleCurrent|selectionBehaviorFlags();
             break;
         }
         case QEvent::MouseButtonPress: {
-            modifiers = static_cast<const QMouseEvent*>(event)->modifiers();
             const Qt::MouseButton button = static_cast<const QMouseEvent*>(event)->button();
             const bool rightButtonPressed = button & Qt::RightButton;
             const bool shiftKeyPressed = modifiers & Qt::ShiftModifier;
@@ -4016,7 +4004,6 @@ QItemSelectionModel::SelectionFlags QAbstractItemViewPrivate::extendedSelectionC
         }
         case QEvent::MouseButtonRelease: {
             // ClearAndSelect on MouseButtonRelease if MouseButtonPress on selected item or empty area
-            modifiers = static_cast<const QMouseEvent*>(event)->modifiers();
             const Qt::MouseButton button = static_cast<const QMouseEvent*>(event)->button();
             const bool rightButtonPressed = button & Qt::RightButton;
             const bool shiftKeyPressed = modifiers & Qt::ShiftModifier;
@@ -4029,7 +4016,6 @@ QItemSelectionModel::SelectionFlags QAbstractItemViewPrivate::extendedSelectionC
         }
         case QEvent::KeyPress: {
             // NoUpdate on Key movement and Ctrl
-            modifiers = static_cast<const QKeyEvent*>(event)->modifiers();
             switch (static_cast<const QKeyEvent*>(event)->key()) {
             case Qt::Key_Backtab:
                 modifiers = modifiers & ~Qt::ShiftModifier; // special case for backtab