[REG 5.10->5.12] Multi-touch screen io: stationary first touch point blocks additional (2nd, 3rd etc) touch point move events.

I have encountered a problem with the handling of multi-touch screen events in both 5.12.0 and 5.12.1 in my project. This problem is not present in earlier versions of Qt in particular 5.10 and earlier. When more than one touch points are applied to a multi-touch screen, Qt::TouchPointMoved events are queued and but not issued for the 2nd 3rd, and, 4th ... touch point drag events when the first touch point is kept stationary. If however the additional touch points are released (lifted from the screen), all the queued TouchPointMoved events are issued one after another all at once catching up all the move events. Also if the first touch point is lifted, the queued touch points are also released all at once catching up the interface. This bug renders multi-touch applications that rely on independent touch point activity inoperable.

This behavior can easily be reproduced using the Qt Creator 4.8.0 interface that is based on 5.12.0 (MSVC 2015, 32 bit) using two fingers to scroll the editor interface. Using one finger from each hand, place them on the screen noting which finger touches the screen first. This first finger will dominate the scroll event. When the first finger moves alone the text editor is correctly scrolled. However moving the second finger alone while keeping the first finger stationary does not scroll the screen. If however the second finger is then lifted after moving it, all the queued move events are released and issued. All the secondary Qt::TouchPointMoved events seem to be recorded accurately, they are just being blocked or buffered until the primary touch point moves or is lifted. As long as the first finger is generating TouchPointMoved events, auxiliary touch point move events are also issued making it seem as if everything is fine.