Tapping with multiple touch screens causes touch press/release state to become mismatched/inconsistent.

The UI is becoming frozen after a certain combination of touches from multiple touchscreens.

This issue is also present in Qt 5.10.

I can reproduce it on an embedded machine with X11, and using Ubuntu 18.10 (with both precompiled Qt deliveries and manually compiled kits).

I have inspected the QAbstractNativeEventFilter for xorg and the events for the touch sequences are correct.

To reproduce:

1. Plug in two touch screen devices (device A and device B).
2. Press and hold on device A.
3. Press/release/press and hold on device B.
4. Release device A.

It doesn't happen every time. With this combination, I can reproduce the issue in about 1/5 tries.

If I alt/tab or change focus to another app, and bring focus back to the Qt/QML app, it works again!