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>

2 files changed, 52 insertions, 17 deletions

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