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  1. Qt
  2. QTBUG-119030

xcb plugin mixes up touch events for multiple windows

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Details

    • Bug
    • Resolution: Unresolved
    • P1: Critical
    • None
    • 6.0, 6.6.0
    • QPA: X11/XCB
    • None

    Description

      Found while testing QTBUG-118909:

      void QXcbConnection::xi2ProcessTouch(void *xiDevEvent, QXcbWindow *platformWindow)
      {
          auto *xiDeviceEvent = reinterpret_cast<xcb_input_touch_begin_event_t *>(xiDevEvent);
          TouchDeviceData *dev = touchDeviceForId(xiDeviceEvent->sourceid);
          Q_ASSERT(dev);
          const bool firstTouch = dev->touchPoints.isEmpty();
          if (xiDeviceEvent->event_type == XCB_INPUT_TOUCH_BEGIN) {
              QWindowSystemInterface::TouchPoint tp;
              tp.id = xiDeviceEvent->detail % INT_MAX; // <-- only one touchpoint in this XI2 event
              tp.state = QEventPoint::State::Pressed;
              tp.pressure = -1.0;
              dev->touchPoints[tp.id] = tp;
          }
      ...
          QWindowSystemInterface::handleTouchEvent(platformWindow->window(), xiDeviceEvent->time, dev->qtTouchDevice, dev->touchPoints.values(), modifiers);
      

      dev->touchPoints.values() is all active touchpoints in all windows. QGuiApplicationPrivate::processTouchEvent() takes the window from the event, not the touchpoint. (QWindowSystemInterface::TouchPoint does not include the window, so we do not have the ability to send one event with touches in multiple windows via QWindowSystemInterface::handleTouchEvent(): it needs to be a separate event for each window. This could change if we start using QEventPoint there instead.) Depending on the QHash order, it might first find the pre-existing touchpoint and assign it a new window.

      Maybe this is a good reason to take care of QTBUG-119029 and start trying to send QEventPoints directly from the plugin to another overload of QWindowSystemInterface::handleTouchEvent(). It will avoid some conversion overhead; QEventPointPrivate has a QWindow*, so an event with touches in multiple windows might be ok; and, QGuiApplicationPrivate::processTouchEvent() already has code to split up the events by window.

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              srutledg Shawn Rutledge
              srutledg Shawn Rutledge
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                Updated:

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