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>

1 files changed, 1 insertions, 2 deletions

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;