WebEngineView's built-in scroll bar stops working after mouse is moved out of the window

Sample code:

import QtQuick 2.12
import QtQuick.Window 2.12
import QtWebEngine 1.6

Window {
    visible: true
    width: 800
    height: 600
    title: qsTr("Test WebEngineView Scroll Bar")
    color: "gray"

    Rectangle {
        anchors.fill: parent
        anchors.margins: 40
        color: "white"

        WebEngineView {
            anchors.fill: parent
            url: "https://www.qt.io"
        }
    }
}

 Steps to reproduce:

1. Run the sample code above on macOS (tested on macOS 10.13.6-17G14033)
2. After the page is loaded, drag the scroll bar
3. While mouse is pressed, move the mouse out of the window
4. Observe the bug

Expected:

• The scroll bar should still follow the mouse's movement

Actual:

• The scroll bar no longer follows the mouse's movement

Extra notes:

• On Windows the scroll bar seems to work as expected, however, we've observed some strange behaviors when the moused moved back to the scrollbar, it may follow the mouse even if the mouse is no longer pressed