In the Qt documentation here https://doc.qt.io/qt-5/qscroller.html it says that, when enabling QScroller::TouchGesture on a widget:

The gesture recognizer will only trigger on touch events. Specifically it will react on single touch points when using a touch screen and dual touch points when using a touchpad.

The bold part doesn't work.

scroller->grabGesture(scrollArea->viewport(),QScroller::LeftMouseButtonGesture);

Works as intended, but

scroller->grabGesture(scrollArea->viewport(), QScroller::TouchGesture);

Doesn't change anything at all

I modified the official image viewer example to use QScroller and test both conditions, one can clearly see that one version works and the other doesn't