The mouse extra buttons such as "back" are incorrectly delivered. In this case "back" is delivered as "middle".

This issue is fixed in Qt 5.10, but should be backported to the LTS.

Here is the fix as implemented in 5.10, it works on 5.6 as well:

/src/plugins/platforms/cocoa/qcocoahelpers.mm

Qt::MouseButton cocoaButton2QtButton(NSInteger buttonNum)
{
    if (buttonNum >= 0 && buttonNum <= 31)
        return Qt::MouseButton(1 << buttonNum);
    return Qt::NoButton;
}