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>

4 files changed, 16 insertions, 6 deletions

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;