Track target widget when wheel events are received

This issue is reproducible on OS X when using a Magic Mouse
or a combination of Magic Trackpad and regular mouse. In these
cases it's possible to start a scrolling gesture on one widget
and move the mouse cursor over another widget.

Although we send the wheel event phase information, we never
made any use of it. This means that a widget would start
scrolling even though it never received a ScrollBegin event.

In this patch, we make sure the scrolling cycle is respected
and that once a widget starts scrolling, it'll be recieving
all the wheel events until a ScrollEnd event reaches the
application.

For those input devices not supporting a proper phase cycle,
we introduce a new (undocumented) phase value, NoScrollPhase.
If the wheel event phase is NoScrollPhase, then we ignore
the current scroll widget and proceed as usual. This value
is the default for wheel events. It's up to the platform
plugin to set the proper phase value according to the data
received from the OS.

Finally, we fix a few of QWheelEvent constructors to properly
initialize the phase and source properties.

Task-number: QTBUG-50199
Change-Id: I3773729a9c757e2d2fcc5100dcd79f0ed26cb808
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>

9 files changed, 74 insertions, 24 deletions

diff --git a/src/corelib/global/qnamespace.h b/src/corelib/global/qnamespace.h
index c4f5415a01..4d0bd6903d 100644
--- a/src/corelib/global/qnamespace.h
+++ b/src/corelib/global/qnamespace.h
@@ -1626,7 +1626,8 @@ public:
     };
 
     enum ScrollPhase {
-        ScrollBegin = 1,
+        NoScrollPhase = 0, // Make public in 5.7 or asap
+        ScrollBegin,
         ScrollUpdate,
         ScrollEnd
     };
diff --git a/src/corelib/global/qnamespace.qdoc b/src/corelib/global/qnamespace.qdoc
index 7c5263ddbd..527bded3c2 100644
--- a/src/corelib/global/qnamespace.qdoc
+++ b/src/corelib/global/qnamespace.qdoc
@@ -3070,6 +3070,8 @@
 
     This enum describes the phase of scrolling.
 
+    \omitvalue NoScrollPhase The input device doesn't support scroll phase.
+
     \value ScrollBegin Scrolling is about to begin, but the scrolling
     distance did not yet change.
 
diff --git a/src/gui/kernel/qevent.cpp b/src/gui/kernel/qevent.cpp
index 4f368e1db3..8abec2b05e 100644
--- a/src/gui/kernel/qevent.cpp
+++ b/src/gui/kernel/qevent.cpp
@@ -727,7 +727,7 @@ QWheelEvent::QWheelEvent(const QPointF &pos, int delta,
                          Qt::MouseButtons buttons, Qt::KeyboardModifiers modifiers,
                          Qt::Orientation orient)
     : QInputEvent(Wheel, modifiers), p(pos), qt4D(delta), qt4O(orient), mouseState(buttons),
-      ph(Qt::ScrollUpdate), src(Qt::MouseEventNotSynthesized)
+      ph(Qt::NoScrollPhase), src(Qt::MouseEventNotSynthesized)
 {
     g = QCursor::pos();
     if (orient == Qt::Vertical)
@@ -762,7 +762,7 @@ QWheelEvent::QWheelEvent(const QPointF &pos, const QPointF& globalPos, int delta
                          Qt::MouseButtons buttons, Qt::KeyboardModifiers modifiers,
                          Qt::Orientation orient)
     : QInputEvent(Wheel, modifiers), p(pos), g(globalPos), qt4D(delta), qt4O(orient), mouseState(buttons),
-      ph(Qt::ScrollUpdate), src(Qt::MouseEventNotSynthesized)
+      ph(Qt::NoScrollPhase), src(Qt::MouseEventNotSynthesized)
 {
     if (orient == Qt::Vertical)
         angleD = QPoint(0, delta);
@@ -798,7 +798,7 @@ QWheelEvent::QWheelEvent(const QPointF &pos, const QPointF& globalPos,
             QPoint pixelDelta, QPoint angleDelta, int qt4Delta, Qt::Orientation qt4Orientation,
             Qt::MouseButtons buttons, Qt::KeyboardModifiers modifiers)
     : QInputEvent(Wheel, modifiers), p(pos), g(globalPos), pixelD(pixelDelta),
-      angleD(angleDelta), qt4D(qt4Delta), qt4O(qt4Orientation), mouseState(buttons), ph(Qt::ScrollUpdate),
+      angleD(angleDelta), qt4D(qt4Delta), qt4O(qt4Orientation), mouseState(buttons), ph(Qt::NoScrollPhase),
       src(Qt::MouseEventNotSynthesized)
 {}
 
diff --git a/src/gui/kernel/qevent.h b/src/gui/kernel/qevent.h
index ed2deda9a0..b407663338 100644
--- a/src/gui/kernel/qevent.h
+++ b/src/gui/kernel/qevent.h
@@ -219,6 +219,8 @@ protected:
     uint ph : 2;
     uint src: 2;
     int reserved : 28;
+
+    friend class QApplication;
 };
 #endif
 
diff --git a/src/gui/kernel/qwindowsysteminterface.h b/src/gui/kernel/qwindowsysteminterface.h
index a27c68649e..20d560f82e 100644
--- a/src/gui/kernel/qwindowsysteminterface.h
+++ b/src/gui/kernel/qwindowsysteminterface.h
@@ -94,8 +94,16 @@ public:
                                        quint32 nativeModifiers,
                                        const QString& text = QString(), bool autorep = false,
                                        ushort count = 1, bool tryShortcutOverride = true);
-    static void handleWheelEvent(QWindow *w, const QPointF & local, const QPointF & global, QPoint pixelDelta, QPoint angleDelta, Qt::KeyboardModifiers mods = Qt::NoModifier, Qt::ScrollPhase phase = Qt::ScrollUpdate, Qt::MouseEventSource source = Qt::MouseEventNotSynthesized);
-    static void handleWheelEvent(QWindow *w, ulong timestamp, const QPointF & local, const QPointF & global, QPoint pixelDelta, QPoint angleDelta, Qt::KeyboardModifiers mods = Qt::NoModifier, Qt::ScrollPhase phase = Qt::ScrollUpdate, Qt::MouseEventSource source = Qt::MouseEventNotSynthesized);
+    static void handleWheelEvent(QWindow *w, const QPointF & local, const QPointF & global,
+                                 QPoint pixelDelta, QPoint angleDelta,
+                                 Qt::KeyboardModifiers mods = Qt::NoModifier,
+                                 Qt::ScrollPhase phase = Qt::NoScrollPhase,
+                                 Qt::MouseEventSource source = Qt::MouseEventNotSynthesized);
+    static void handleWheelEvent(QWindow *w, ulong timestamp, const QPointF & local, const QPointF & global,
+                                 QPoint pixelDelta, QPoint angleDelta,
+                                 Qt::KeyboardModifiers mods = Qt::NoModifier,
+                                 Qt::ScrollPhase phase = Qt::NoScrollPhase,
+                                 Qt::MouseEventSource source = Qt::MouseEventNotSynthesized);
 
     // Wheel event compatibility functions. Will be removed: do not use.
     static void handleWheelEvent(QWindow *w, const QPointF & local, const QPointF & global, int d, Qt::Orientation o, Qt::KeyboardModifiers mods = Qt::NoModifier);
diff --git a/src/gui/kernel/qwindowsysteminterface_p.h b/src/gui/kernel/qwindowsysteminterface_p.h
index e48d1e965b..c2569f29cd 100644
--- a/src/gui/kernel/qwindowsysteminterface_p.h
+++ b/src/gui/kernel/qwindowsysteminterface_p.h
@@ -236,7 +236,7 @@ public:
     class WheelEvent : public InputEvent {
     public:
         WheelEvent(QWindow *w, ulong time, const QPointF & local, const QPointF & global, QPoint pixelD, QPoint angleD, int qt4D, Qt::Orientation qt4O,
-                   Qt::KeyboardModifiers mods, Qt::ScrollPhase phase = Qt::ScrollUpdate, Qt::MouseEventSource src = Qt::MouseEventNotSynthesized)
+                   Qt::KeyboardModifiers mods, Qt::ScrollPhase phase = Qt::NoScrollPhase, Qt::MouseEventSource src = Qt::MouseEventNotSynthesized)
             : InputEvent(w, time, Wheel, mods), pixelDelta(pixelD), angleDelta(angleD), qt4Delta(qt4D), qt4Orientation(qt4O), localPos(local), globalPos(global), phase(phase), source(src) { }
         QPoint pixelDelta;
         QPoint angleDelta;
diff --git a/src/plugins/platforms/cocoa/qnsview.mm b/src/plugins/platforms/cocoa/qnsview.mm
index 0d80333e65..05dcbd33ad 100644
--- a/src/plugins/platforms/cocoa/qnsview.mm
+++ b/src/plugins/platforms/cocoa/qnsview.mm
@@ -1405,6 +1405,8 @@ static QTabletEvent::TabletDevice wacomTabletDevice(NSEvent *theEvent)
                momentumPhase == NSEventPhaseEnded || momentumPhase == NSEventPhaseCancelled) {
         ph = Qt::ScrollEnd;
         m_scrolling = false;
+    } else if (phase == NSEventPhaseNone && momentumPhase == NSEventPhaseNone) {
+        ph = Qt::NoScrollPhase;
     }
 
     QWindowSystemInterface::handleWheelEvent(m_window, qt_timestamp, qt_windowPoint, qt_screenPoint, pixelDelta, angleDelta, currentWheelModifiers, ph, source);
diff --git a/src/widgets/kernel/qapplication.cpp b/src/widgets/kernel/qapplication.cpp
index 3ab477ccfb..7c5a0da0c1 100644
--- a/src/widgets/kernel/qapplication.cpp
+++ b/src/widgets/kernel/qapplication.cpp
@@ -417,6 +417,7 @@ QWidget *QApplicationPrivate::hidden_focus_widget = 0; // will get keyboard inpu
 QWidget *QApplicationPrivate::active_window = 0;        // toplevel with keyboard focus
 #ifndef QT_NO_WHEELEVENT
 int QApplicationPrivate::wheel_scroll_lines;   // number of lines to scroll
+QWidget *QApplicationPrivate::wheel_widget = Q_NULLPTR;
 #endif
 bool qt_in_tab_key_event = false;
 int qt_antialiasing_threshold = -1;
@@ -3309,7 +3310,6 @@ bool QApplication::notify(QObject *receiver, QEvent *e)
     case QEvent::Wheel:
         {
             QWidget* w = static_cast<QWidget *>(receiver);
-            QWheelEvent* wheel = static_cast<QWheelEvent*>(e);
 
             // QTBUG-40656, QTBUG-42731: ignore wheel events when a popup (QComboBox) is open.
             if (const QWidget *popup = QApplication::activePopupWidget()) {
@@ -3317,27 +3317,61 @@ bool QApplication::notify(QObject *receiver, QEvent *e)
                     return true;
             }
 
-            QPoint relpos = wheel->pos();
-            bool eventAccepted = wheel->isAccepted();
+            QWheelEvent* wheel = static_cast<QWheelEvent*>(e);
+            const bool spontaneous = wheel->spontaneous();
+            const Qt::ScrollPhase phase = wheel->phase();
 
-            if (e->spontaneous() && wheel->phase() == Qt::ScrollUpdate)
-                QApplicationPrivate::giveFocusAccordingToFocusPolicy(w, e, relpos);
+            if (phase == Qt::NoScrollPhase || phase == Qt::ScrollBegin
+                || (phase == Qt::ScrollUpdate && !QApplicationPrivate::wheel_widget)) {
+
+                if (spontaneous && phase == Qt::ScrollBegin)
+                    QApplicationPrivate::wheel_widget = Q_NULLPTR;
+
+                const QPoint &relpos = wheel->pos();
+
+                if (spontaneous && (phase == Qt::NoScrollPhase || phase == Qt::ScrollUpdate))
+                    QApplicationPrivate::giveFocusAccordingToFocusPolicy(w, e, relpos);
 
-            while (w) {
                 QWheelEvent we(relpos, wheel->globalPos(), wheel->pixelDelta(), wheel->angleDelta(), wheel->delta(), wheel->orientation(), wheel->buttons(),
-                               wheel->modifiers(), wheel->phase(), wheel->source());
-                we.spont = wheel->spontaneous();
-                res = d->notify_helper(w, w == receiver ? wheel : &we);
-                eventAccepted = ((w == receiver) ? wheel : &we)->isAccepted();
-                e->spont = false;
-                if ((res && eventAccepted)
-                    || w->isWindow() || w->testAttribute(Qt::WA_NoMousePropagation))
-                    break;
+                               wheel->modifiers(), phase, wheel->source());
+                bool eventAccepted;
+                while (w) {
+                    we.spont = spontaneous && w == receiver;
+                    we.ignore();
+                    res = d->notify_helper(w, &we);
+                    eventAccepted = we.isAccepted();
+                    if (res && eventAccepted) {
+                        if (spontaneous && phase != Qt::NoScrollPhase)
+                            QApplicationPrivate::wheel_widget = w;
+                        break;
+                    }
+                    if (w->isWindow() || w->testAttribute(Qt::WA_NoMousePropagation))
+                        break;
 
-                relpos += w->pos();
-                w = w->parentWidget();
+                    we.p += w->pos();
+                    w = w->parentWidget();
+                }
+                wheel->setAccepted(eventAccepted);
+            } else if (QApplicationPrivate::wheel_widget) {
+                if (!spontaneous) {
+                    // wheel_widget may forward the wheel event to a delegate widget,
+                    // either directly or indirectly (e.g. QAbstractScrollArea will
+                    // forward to its QScrollBars through viewportEvent()). In that
+                    // case, the event will not be spontaneous but synthesized, so
+                    // we can send it straigth to the receiver.
+                    d->notify_helper(w, wheel);
+                } else {
+                    const QPoint &relpos = QApplicationPrivate::wheel_widget->mapFromGlobal(wheel->globalPos());
+                    QWheelEvent we(relpos, wheel->globalPos(), wheel->pixelDelta(), wheel->angleDelta(), wheel->delta(), wheel->orientation(), wheel->buttons(),
+                                   wheel->modifiers(), wheel->phase(), wheel->source());
+                    we.spont = true;
+                    we.ignore();
+                    d->notify_helper(QApplicationPrivate::wheel_widget, &we);
+                    wheel->setAccepted(we.isAccepted());
+                    if (phase == Qt::ScrollEnd)
+                        QApplicationPrivate::wheel_widget = Q_NULLPTR;
+                }
             }
-            wheel->setAccepted(eventAccepted);
         }
         break;
 #endif
diff --git a/src/widgets/kernel/qapplication_p.h b/src/widgets/kernel/qapplication_p.h
index 75d86a5eba..4cce2d84e8 100644
--- a/src/widgets/kernel/qapplication_p.h
+++ b/src/widgets/kernel/qapplication_p.h
@@ -202,6 +202,7 @@ public:
     static QWidget *active_window;
 #ifndef QT_NO_WHEELEVENT
     static int  wheel_scroll_lines;
+    static QWidget *wheel_widget;
 #endif
 
     static int enabledAnimations; // Combination of QPlatformTheme::UiEffect