add cross-platform tablet->mouse event synth; enable on Android

It's urgent to fix the issue that on Android, it became impossible
to interact with any widget or MouseArea which handles only mouse
events but not tablet events, using the stylus, because stylus events
are sent only as QTabletEvents.  Before 5.6 (change
01d78ba86a631386a4d47b7c12d2a359da28f517) they were sent as
touch events, and mouse events were synthesized from those.  Whereas
on other platforms, every QTabletEvent is followed by a synthesized
QMouseEvent.

This fix proceeds in the direction that event synthesis should be done
in cross-platform code so that platform plugins don't have to repeat it,
following the same pattern as for touch->mouse synthesis.  Just as
in that case, the application can disable it, and the platform plugin
can also report that it's unnecessary for Qt to do the synthesis
because the platform already does.

So QTBUG-51618 is fixed, but QTBUG-47007 requires us to remove the
tablet->mouse synthesis from all platform plugins, because the plugin
does not know whether the tablet event was accepted or not, so it does
not have enough information to decide whether to synthesize a mouse
event.  Synthesis has been unconditional until now, which contradicts
what the documentation says: the mouse event should be sent only if
the tablet event is NOT accepted.  We can now gradually make this
promise come true.

[ChangeLog][QtCore][Tablet support] A synthetic mouse event will
no longer be sent after every QTabletEvent, only after those which are
not accepted (as documented).

Task-number: QTBUG-47007
Task-number: QTBUG-51618
Change-Id: I99404e0c2b39bbca4377be6fd48e0c6b20338466
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

examples/widgets/widgets/tablet/tabletcanvas.cpp

@@ -131,6 +131,7 @@ void TabletCanvas::tabletEvent(QTabletEvent *event)
         default:
             break;
     }
+    event->accept();
     update();
 }
 //! [3]

src/corelib/global/qnamespace.h

@@ -507,6 +507,7 @@ public:
         AA_DisableHighDpiScaling = 21,
         AA_UseStyleSheetPropagationInWidgetStyles = 22, // ### Qt 6: remove me
         AA_DontUseNativeDialogs = 23,
+        AA_SynthesizeMouseForUnhandledTabletEvents = 24,
 
         // Add new attributes before this line
         AA_AttributeCount

src/corelib/kernel/qcoreapplication.cpp

@@ -325,7 +325,9 @@ Qt::HANDLE qt_application_thread_id = 0;
 #endif // QT_NO_QOBJECT
 
 QCoreApplication *QCoreApplication::self = 0;
-uint QCoreApplicationPrivate::attribs = (1 << Qt::AA_SynthesizeMouseForUnhandledTouchEvents);
+uint QCoreApplicationPrivate::attribs =
+    (1 << Qt::AA_SynthesizeMouseForUnhandledTouchEvents) |
+    (1 << Qt::AA_SynthesizeMouseForUnhandledTabletEvents);
 
 struct QCoreApplicationData {
     QCoreApplicationData() Q_DECL_NOTHROW {

src/gui/kernel/qguiapplication.cpp

@@ -2300,9 +2300,17 @@ void QGuiApplicationPrivate::processTabletEvent(QWindowSystemInterfacePrivate::T
                     e->device, e->pointerType, e->pressure, e->xTilt, e->yTilt,
                     e->tangentialPressure, e->rotation, e->z,
                     e->modifiers, e->uid, button, e->buttons);
+    ev.setAccepted(false);
     ev.setTimestamp(e->timestamp);
     QGuiApplication::sendSpontaneousEvent(window, &ev);
     pointData.state = e->buttons;
+    if (!ev.isAccepted() && !QWindowSystemInterfacePrivate::TabletEvent::platformSynthesizesMouse
+            && qApp->testAttribute(Qt::AA_SynthesizeMouseForUnhandledTabletEvents)) {
+        QWindowSystemInterfacePrivate::MouseEvent fake(window, e->timestamp, e->local, e->global,
+                                                       e->buttons, e->modifiers, Qt::MouseEventSynthesizedByQt);
+        fake.flags |= QWindowSystemInterfacePrivate::WindowSystemEvent::Synthetic;
+        processMouseEvent(&fake);
+    }
 #else
     Q_UNUSED(e)
 #endif

src/gui/kernel/qwindowsysteminterface.cpp

@@ -54,6 +54,7 @@ QT_BEGIN_NAMESPACE
 
 QElapsedTimer QWindowSystemInterfacePrivate::eventTime;
 bool QWindowSystemInterfacePrivate::synchronousWindowSystemEvents = false;
+bool QWindowSystemInterfacePrivate::TabletEvent::platformSynthesizesMouse = true;
 QWaitCondition QWindowSystemInterfacePrivate::eventsFlushed;
 QMutex QWindowSystemInterfacePrivate::flushEventMutex;
 QAtomicInt QWindowSystemInterfacePrivate::eventAccepted;
@@ -945,4 +946,9 @@ QWindowSystemInterfacePrivate::WheelEvent::WheelEvent(QWindow *w, ulong time, co
 {
 }
 
+void QWindowSystemInterfacePrivate::TabletEvent::setPlatformSynthesizesMouse(bool v)
+{
+    platformSynthesizesMouse = v;
+}
+
 QT_END_NAMESPACE

src/gui/kernel/qwindowsysteminterface_p.h

@@ -352,6 +352,7 @@ public:
                                       int device, int pointerType, Qt::MouseButtons buttons, qreal pressure, int xTilt, int yTilt,
                                       qreal tangentialPressure, qreal rotation, int z, qint64 uid,
                                       Qt::KeyboardModifiers modifiers = Qt::NoModifier);
+        static void setPlatformSynthesizesMouse(bool v);
 
         TabletEvent(QWindow *w, ulong time, const QPointF &local, const QPointF &global,
                     int device, int pointerType, Qt::MouseButtons b, qreal pressure, int xTilt, int yTilt, qreal tpressure,
@@ -372,6 +373,7 @@ public:
         qreal rotation;
         int z;
         qint64 uid;
+        static bool platformSynthesizesMouse;
     };
 
     class TabletEnterProximityEvent : public InputEvent {

src/plugins/platforms/android/androidjnimain.cpp

@@ -864,6 +864,7 @@ Q_DECL_EXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void */*reserved*/)
         __android_log_print(ANDROID_LOG_FATAL, "Qt", "registerNatives failed");
         return -1;
     }
+    QWindowSystemInterfacePrivate::TabletEvent::setPlatformSynthesizesMouse(false);
 
     m_javaVM = vm;
     return JNI_VERSION_1_4;

src/widgets/kernel/qwidgetwindow.cpp

@@ -956,6 +956,7 @@ void QWidgetWindow::handleTabletEvent(QTabletEvent *event)
                         event->rotation(), event->z(), event->modifiers(), event->uniqueId(), event->button(), event->buttons());
         ev.setTimestamp(event->timestamp());
         QGuiApplication::sendSpontaneousEvent(qt_tablet_target, &ev);
+        event->setAccepted(ev.isAccepted());
     }
 
     if (event->type() == QEvent::TabletRelease && event->buttons() == Qt::NoButton)