Reload lastScrollPosition on window enter

We need the lastScrollPosition position to calculated scrolling deltas
on scroll events. Since the position is tied to the device and might
have changed while scroll events were send to other applications we need
to reload the value when mouse focus reenters our application.

Task-number: QTBUG-38274
Change-Id: Ic166648f8e7ae486288cbed339a057e3faa1ef2d
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>

src/plugins/platforms/xcb/qxcbconnection.h

@@ -454,6 +454,10 @@ public:
 
     QXcbSystemTrayTracker *systemTrayTracker();
 
+#ifdef XCB_USE_XINPUT2
+    void handleEnterEvent(const xcb_enter_notify_event_t *);
+#endif
+
 private slots:
     void processXcbEvents();
 

src/plugins/platforms/xcb/qxcbconnection_xi2.cpp

@@ -524,6 +524,35 @@ void QXcbConnection::xi2HandleEvent(xcb_ge_event_t *event)
     }
 }
 
+void QXcbConnection::handleEnterEvent(const xcb_enter_notify_event_t *)
+{
+#ifdef XCB_USE_XINPUT21
+    QHash<int, ScrollingDevice>::iterator it = m_scrollingDevices.begin();
+    const QHash<int, ScrollingDevice>::iterator end = m_scrollingDevices.end();
+    while (it != end) {
+        ScrollingDevice& scrollingDevice = it.value();
+        int nrDevices = 0;
+        XIDeviceInfo* xiDeviceInfo = XIQueryDevice(static_cast<Display *>(m_xlib_display), scrollingDevice.deviceId, &nrDevices);
+        if (nrDevices <= 0) {
+            it = m_scrollingDevices.erase(it);
+            continue;
+        }
+        for (int c = 0; c < xiDeviceInfo->num_classes; ++c) {
+            if (xiDeviceInfo->classes[c]->type == XIValuatorClass) {
+                XIValuatorClassInfo *vci = reinterpret_cast<XIValuatorClassInfo *>(xiDeviceInfo->classes[c]);
+                const int valuatorAtom = qatom(vci->label);
+                if (valuatorAtom == QXcbAtom::RelHorizScroll || valuatorAtom == QXcbAtom::RelHorizWheel)
+                    scrollingDevice.lastScrollPosition.setX(vci->value);
+                else if (valuatorAtom == QXcbAtom::RelVertScroll || valuatorAtom == QXcbAtom::RelVertWheel)
+                    scrollingDevice.lastScrollPosition.setY(vci->value);
+            }
+        }
+        XIFreeDeviceInfo(xiDeviceInfo);
+        ++it;
+    }
+#endif
+}
+
 void QXcbConnection::xi2HandleScrollEvent(void *event, ScrollingDevice &scrollingDevice)
 {
 #ifdef XCB_USE_XINPUT21

src/plugins/platforms/xcb/qxcbwindow.cpp

@@ -1778,6 +1778,9 @@ public:
 void QXcbWindow::handleEnterNotifyEvent(const xcb_enter_notify_event_t *event)
 {
     connection()->setTime(event->time);
+#ifdef XCB_USE_XINPUT2
+    connection()->handleEnterEvent(event);
+#endif
 
     if ((event->mode != XCB_NOTIFY_MODE_NORMAL && event->mode != XCB_NOTIFY_MODE_UNGRAB)
         || event->detail == XCB_NOTIFY_DETAIL_VIRTUAL