QQuickWidget buttons triggered twice on touch press when stylus is hovering on another widget

Context: https://bugs.kde.org/show_bug.cgi?id=437196

When a pen stylus is hovering and moving on another QWidget, pressing a QML button in a QQuickWidget may cause it to be triggered twice.

Attached is a test project. To reproduce:

• Hover pen stylus on the right-hand area and move it around continuously.
• Press the button on the left-hand side using touchscreen.
• Observe the "Button is pressed" message in the console.

Sometimes you can see two "Button is pressed" messages printed consecutively.

My suspect is the change in https://codereview.qt-project.org/c/qt/qtbase/+/267594, which was supposed to fix QTBUG-76617. The code checks for the last WM_POINTERUPDATE / WM_POINTER* pointer type to set Qt::MouseEventSynthesizedBySystem, which in theory could fail to work properly if the WM_POINTERUP and WM_LBUTTONDOWN messages produced from the touch press event is interleaved with a WM_POINTERUPDATE message from the pen movement.

It probably is still reproducible on Qt 5.15.* and 6 but I cannot test it locally for now.