5 files changed, 28 insertions, 3 deletions
diff --git a/src/gui/kernel/qevent.cpp b/src/gui/kernel/qevent.cpp
index 8abec2b05e..9281744692 100644
--- a/src/gui/kernel/qevent.cpp
+++ b/src/gui/kernel/qevent.cpp
@@ -729,6 +729,8 @@ QWheelEvent::QWheelEvent(const QPointF &pos, int delta,
     : QInputEvent(Wheel, modifiers), p(pos), qt4D(delta), qt4O(orient), mouseState(buttons),
       ph(Qt::NoScrollPhase), src(Qt::MouseEventNotSynthesized)
 {
+    if (!QGuiApplicationPrivate::scrollNoPhaseAllowed)
+        ph = Qt::ScrollUpdate;
     g = QCursor::pos();
     if (orient == Qt::Vertical)
         angleD = QPoint(0, delta);
@@ -764,6 +766,8 @@ QWheelEvent::QWheelEvent(const QPointF &pos, const QPointF& globalPos, int delta
     : QInputEvent(Wheel, modifiers), p(pos), g(globalPos), qt4D(delta), qt4O(orient), mouseState(buttons),
       ph(Qt::NoScrollPhase), src(Qt::MouseEventNotSynthesized)
 {
+    if (!QGuiApplicationPrivate::scrollNoPhaseAllowed)
+        ph = Qt::ScrollUpdate;
     if (orient == Qt::Vertical)
         angleD = QPoint(0, delta);
     else
@@ -800,7 +804,10 @@ QWheelEvent::QWheelEvent(const QPointF &pos, const QPointF& globalPos,
     : QInputEvent(Wheel, modifiers), p(pos), g(globalPos), pixelD(pixelDelta),
       angleD(angleDelta), qt4D(qt4Delta), qt4O(qt4Orientation), mouseState(buttons), ph(Qt::NoScrollPhase),
       src(Qt::MouseEventNotSynthesized)
-{}
+{
+    if (!QGuiApplicationPrivate::scrollNoPhaseAllowed)
+        ph = Qt::ScrollUpdate;
+}
 
 /*!
     Constructs a wheel event object.
diff --git a/src/gui/kernel/qguiapplication.cpp b/src/gui/kernel/qguiapplication.cpp
index 3fe6698955..3808bec8a4 100644
--- a/src/gui/kernel/qguiapplication.cpp
+++ b/src/gui/kernel/qguiapplication.cpp
@@ -186,6 +186,9 @@ bool QGuiApplicationPrivate::obey_desktop_settings = true;
 
 QInputDeviceManager *QGuiApplicationPrivate::m_inputDeviceManager = 0;
 
+// enable the fix for QTBUG-50199; TODO remove this check in 5.7
+bool QGuiApplicationPrivate::scrollNoPhaseAllowed = false;
+
 static qreal fontSmoothingGamma = 1.7;
 
 extern void qRegisterGuiVariant();
@@ -1412,6 +1415,8 @@ void QGuiApplicationPrivate::init()
 
     if (layout_direction == Qt::LayoutDirectionAuto || force_reverse)
         QGuiApplication::setLayoutDirection(qt_detectRTLLanguage() ? Qt::RightToLeft : Qt::LeftToRight);
+
+    scrollNoPhaseAllowed = qEnvironmentVariableIsSet("QT_ENABLE_MOUSE_WHEEL_TRACKING");
 }
 
 extern void qt_cleanupFontDatabase();
diff --git a/src/gui/kernel/qguiapplication_p.h b/src/gui/kernel/qguiapplication_p.h
index 7c7da9790b..9c8b655c71 100644
--- a/src/gui/kernel/qguiapplication_p.h
+++ b/src/gui/kernel/qguiapplication_p.h
@@ -285,6 +285,9 @@ public:
 
     static void setApplicationState(Qt::ApplicationState state, bool forcePropagate = false);
 
+    // enable the fix for QTBUG-50199; TODO remove this check in 5.7
+    static bool scrollNoPhaseAllowed;
+
 protected:
     virtual void notifyThemeChanged();
     bool tryCloseRemainingWindows(QWindowList processedWindows);
diff --git a/src/gui/kernel/qwindowsysteminterface.cpp b/src/gui/kernel/qwindowsysteminterface.cpp
index 841e4f8f41..cae976098c 100644
--- a/src/gui/kernel/qwindowsysteminterface.cpp
+++ b/src/gui/kernel/qwindowsysteminterface.cpp
@@ -311,6 +311,9 @@ void QWindowSystemInterface::handleWheelEvent(QWindow *w, const QPointF & local,
 
 void QWindowSystemInterface::handleWheelEvent(QWindow *tlw, ulong timestamp, const QPointF & local, const QPointF & global, QPoint pixelDelta, QPoint angleDelta, Qt::KeyboardModifiers mods, Qt::ScrollPhase phase, Qt::MouseEventSource source)
 {
+    if (!QGuiApplicationPrivate::scrollNoPhaseAllowed && phase == Qt::NoScrollPhase)
+        phase = Qt::ScrollUpdate;
+
     // Qt 4 sends two separate wheel events for horizontal and vertical
     // deltas. For Qt 5 we want to send the deltas in one event, but at the
     // same time preserve source and behavior compatibility with Qt 4.
@@ -930,5 +933,13 @@ bool QWindowSystemEventHandler::sendEvent(QWindowSystemInterfacePrivate::WindowS
     return true;
 }
 
+QWindowSystemInterfacePrivate::WheelEvent::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::MouseEventSource src)
+    : InputEvent(w, time, Wheel, mods), pixelDelta(pixelD), angleDelta(angleD), qt4Delta(qt4D),
+      qt4Orientation(qt4O), localPos(local), globalPos(global),
+      phase(!QGuiApplicationPrivate::scrollNoPhaseAllowed && phase == Qt::NoScrollPhase ? Qt::ScrollUpdate : phase),
+      source(src)
+{
+}
 
 QT_END_NAMESPACE
diff --git a/src/gui/kernel/qwindowsysteminterface_p.h b/src/gui/kernel/qwindowsysteminterface_p.h
index c2569f29cd..b3c6d0d96d 100644
--- a/src/gui/kernel/qwindowsysteminterface_p.h
+++ b/src/gui/kernel/qwindowsysteminterface_p.h
@@ -236,8 +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::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) { }
+                   Qt::KeyboardModifiers mods, Qt::ScrollPhase phase = Qt::NoScrollPhase, Qt::MouseEventSource src = Qt::MouseEventNotSynthesized);
         QPoint pixelDelta;
         QPoint angleDelta;
         int qt4Delta;