Jittery movement of QScrollArea from custom touchscreen driver - c++

Question

So a few years ago, I wrote a custom touchscreen driver specifically for a particular application which ran on a Scientific Linux 6.4 (CentOS 6 based) OS, which did not have native touch support, but the kernel supported touch events, so I was able to directly read the raw data from the touchscreen in /dev/input/event* and read the event data to generate mouse events with Qt to control the application to mimic a multi-touch touchscreen. More recently, we've finally migrated to RedHat 8.4, but I had to disable the native touch driver, because as far as I know, the native Qt touch events didn't allow the same degree of control over the application that my driver did.
Recently during a trial, a technician reported that when using the touchscreen to manipulate one of the application's image display areas, the movement was very "jittery". The image display area is simply a QScrollArea with a QImage inside, and hidden scroll bars. The way it works is that on a mouseMoveEvent, it manipulates the scrollbars according to the delta value on the mouse event.
void PanArea::mouseMoveEvent(QMouseEvent* e)
{
SafeStr str;
if (m_pw)
{
if (m_disable_panning == false &&
e->buttons().testFlag(Qt::RightButton))
{
QPoint delta = e->pos() - m_last_pos;
horizontalScrollBar()->setValue(horizontalScrollBar()->value() -
delta.x());
verticalScrollBar()->setValue(verticalScrollBar()->value() -
delta.y());
m_last_pos = e->pos();
emit signalScrollPositionChanged(getScrollPosition());
// This force update has been added because
// when fast panning cause black to appear within the image
// because some pan widget updates were being skipped.
// Performance seems acceptable with this here.
m_pw->update();
}
else if (...)
{
// irrelevant code removed to save space
}
}
}
This works fine when simply right-click dragging on the scroll area. And then here is the relevant function that dispatches the mouse press and mouse move events from the touchscreen driver:
void InputHandler::panStart(TouchPoint* tp, QWidget* widget)
{
QPoint pos(tp->cx(), tp->cy());
QWidget* target = widget;
if (target == NULL) target = QApplication::widgetAt(pos);
if (target != NULL)
{
QPoint local = target->mapFromGlobal(pos);
QMouseEvent* press =
new QMouseEvent(QEvent::MouseButtonPress, local, pos,
Qt::RightButton, Qt::RightButton, Qt::NoModifier);
QApplication::postEvent(widget, press);
}
}
void InputHandler::panMove(TouchPoint* tp, QWidget* widget)
{
QPoint pos(tp->cx(), tp->cy());
QWidget* target = widget;
if (target == NULL) target = QApplication::widgetAt(pos);
if (target != NULL)
{
QPoint local = target->mapFromGlobal(pos);
QMouseEvent* move =
new QMouseEvent(QEvent::MouseMove, local, pos, Qt::NoButton,
Qt::RightButton, Qt::NoModifier);
QApplication::postEvent(widget, move);
}
}
When I touch and drag on the widget, it DOES move, but it jumps all over the place as if I were rapidly dragging it in random directions.
Is there some caveat of how Qt5 mouse events work that could explain this behavior? Or something to do with the way the widget is moving the image around?