The setup is simplified this: Flickable { Rectangle { MouseArea {} } }. When the user pressed the mouse, moves 5 pixel and releases the mouse the mouse area get's a press/cancel sequenze. The next press/release is grabbed by the flickable. I have attached a short unit test with different good and bad sequences.

(mouse positioned on mouse area)
// sequenze 1
1. mouse down
2. mouse move 5px # more pixels will also be bad, less pixels will be okay
3. mouse up
// sequenze 2: will be grabbed away by the flickable
4. mouse down
5. mouse up

The distance is direct depending on the QApplication::startDragDistance().

In the bad case the scene retains a mouseGrabberItem() which should not be the case in my understanding as after the release the next click should go to the mouse area.