In the following example, we have outer/inner MouseArea of the same size,
and the outer MouseArea handles drag events.

If we grab the edge of the inner MouseArea and drag it towards outside of
the outer MouseArea at least drag.threshold px, dragging doesn't start.
That's probably because the outer MouseArea doesn't receive child mouse events
outside of the item rect, and the drag doesn't start until mouse moves drag.threshold px.

        MouseArea {
            id: outerArea
            drag.filterChildren: true
            drag.target: panel
            drag.axis: Drag.XAxis | Drag.YAxis
            drag.threshold: 10

            // This may be a TabButton in real-world application.
            MouseArea {
                id: innerArea
                anchors.fill: parent
            }
        }

(full source code is attached)

STR:
1. qmlscene drag-filter-children-edge-case.qml
2. click bottom edge and drag it down.

Workaround:
Expand the outer MouseArea while containsPress && !drag.active to receive mouse events.