With an Apple Magic Trackpad:

1. Press a modifier such as the Option key
2. Use the 2 finger gesture to produce scroll wheel events (either horizontally or vertically)
3. Move fast enough and lift your fingers (flick it)

At step 3, momentum generates new scroll wheel events after lifting your fingers.

Between 2-3, we see the Option key modifier
After step 3, we receive scroll wheel events with no modifier set.

Impact: if using the modifier is supposed to produce a different effect,
then flicking alternates from the intended effect back to the no-modifier case.

Note: this does not happen with a regular scroll wheel mouse (such as a pc mouse) because momentum does not generate further scroll events

qtbase/src/plugins/platforms/cocoa/qnsview.mm seems like a nice place to start with to investigate.

With the app provided in attachment the output is:

–

1. step 2
   qml: wheel: 67108864, QPoint(0, -1)
   qml: wheel: 67108864, QPoint(3, 0)
   qml: wheel: 67108864, QPoint(8, 1)
   qml: wheel: 67108864, QPoint(33, 1)
   qml: wheel: 67108864, QPoint(16, 0)
   qml: wheel: 67108864, QPoint(0, 0)
   qml: wheel: 67108864, QPoint(0, 0)
2. step 3
   qml: wheel: 0, QPoint(29, 0)
   qml: wheel: 0, QPoint(33, 0)
   qml: wheel: 0, QPoint(34, 0)
   qml: wheel: 0, QPoint(33, 0)