QWheelEvent: make NoScrollPhase opt-in

The fix for QTBUG-50199 involves adding an undocumented enum value
which can be returned from QWheelEvent::phase().  This will be quite
unexpected for applications that use it, which work fine with 5.6.0
and then start receiving this new phase value in 5.6.1.  So it should
not happen by default.  Set the env variable
QT_ENABLE_MOUSE_WHEEL_TRACKING to enable this functionality.
In 5.7 it will be default behavior.  But in 5.6 the default
behavior is as it was before: if you use a conventional mouse wheel,
the phase stays at ScrollUpdate continuously.

[ChangeLog][QtCore] QWheelEvent::phase() returns 0 rather than
Qt::ScrollUpdate when the wheel event comes from an actual
non-emulated mouse wheel and the environment variable
QT_ENABLE_MOUSE_WHEEL_TRACKING is set.  In Qt 5.6, this is required
to enable the fix for QTBUG-50199.

Change-Id: Ieb2152ff767df24c42730d201235d1225aaec832
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@theqtcompany.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>

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;