Why does QScrollArea scroll on multi-touch even if nobody told it to?

nobru

I'm using a multi-touch monitor (EIZO DuraVision FDF2382WT) with Lubuntu 18.04. I have a QScrollArea with a QLabel as widget. The QLabel has a QPixmap. Touching it with two or more fingers scrolls the picture. Why does this happen? According to my understanding of the documentation QTouchEvent handling is opt-in, except for QWindows which always receive them.

I've tried setting AA_SynthesizeMouseForUnhandledTouchEvents to False, setting WA_AcceptTouchEvents for the QScrollArea and QLabel to both True and False. I saw no changes. I've tried reimplementing QScrollArea.event, QScrollArea.scroll, QLabel.event and the scroll bar setValue to log the calls, but nothing is printed.

import sys

from PyQt5 import Qt

qapp = Qt.QApplication(sys.argv)

picture = Qt.QPixmap('picture.png')

qlab = Qt.QLabel()
qlab.setPixmap(picture)

qsa = Qt.QScrollArea()
qsa.setWidget(qlab)
qsa.setGeometry(10, 30, 200, 200)
qsa.show()

sys.exit(qapp.exec_())

I was expecting the picture to stay in place, instead it is dragged. Nothing happens when dragging with the mouse or with just one finger.

nobru

Finally I've solved this: it was gestures!
I brutally disabled them with something like

import sys

from PyQt5 import Qt

qapp = Qt.QApplication(sys.argv)

picture = Qt.QPixmap('/home/bruno/Pictures/ABCDEFGH_red_on_green.png')

qlab = Qt.QLabel()
qlab.setPixmap(picture)

MyScrollArea = Qt.QScrollArea

def event(self, event):
    print('DEBUG: event type {}'.format(event.type()))
    if event.type() in (Qt.QEvent.Gesture, Qt.QEvent.GestureOverride):
        event.accept()
        return True
    else:
        return super(MyScrollArea, self).event(event)

MyScrollArea.event = event

qsa = MyScrollArea()
qsa.setWidget(qlab)
qsa.setGeometry(10, 30, 200, 200)
qsa.show()

sys.exit(qapp.exec_())