qtdiag:
=======
Screens: 2, High DPI scaling: active
# 0 "\\.\DISPLAY2" Depth: 32 Primary: yes
  Geometry: 1920x1200+0+0 Available: 1920x1160+0+0
  Virtual geometry: 5760x1200-3840+0 Available: 5760x1160-3840+0
Prmary orientation: 2 Orientation: 2 Native orientation: 0 OrientationUpdateMask: 0

# 1 "\\.\DISPLAY1" Depth: 32 Primary: no
  Geometry: 1920x1080-3840+0 (native: 3840x2160-3840+0) Available: 1920x1040-3840+0
  
=============================

>> Native child: No context menu, QContextMenuEvent::globalPos is off
5.12.0 windows/Windows 10 (10.0) ()
PE native winId: 0x 0x1f04fe
"5.12.0 windows/Windows 10 (10.0) \"\\\\.\\DISPLAY1\" 1920x1080-3840+0, DPR=2, windowsvista"
MainWindow::dumpGeom \\.\DISPLAY1 geom= QRect(-3196,294 256x214) dpr= 2
PlainTextEdit::contextMenuEvent # 0 scr= \\.\DISPLAY1 QRect(-3840,0 1920x1080) dpr= 2
  event->pos= QPoint(80,56) e->globalPos= QPoint(-1195,373) cursor= QPoint(4,10) global-cursor= QPoint(-3192,326)

>> No native child: works
C:\dev\projects\qtbug71368_512>debug\qtbug71368.exe   -w
5.12.0 windows/Windows 10 (10.0) (-w)
"5.12.0 windows/Windows 10 (10.0) \"\\\\.\\DISPLAY1\" 1920x1080-3840+0, DPR=2, windowsvista"
MainWindow::dumpGeom \\.\DISPLAY1 geom= QRect(-3196,294 256x214) dpr= 2
PlainTextEdit::contextMenuEvent # 0 scr= \\.\DISPLAY1 QRect(-3840,0 1920x1080) dpr= 2
  event->pos= QPoint(77,72) e->globalPos= QPoint(-3118,389) cursor= QPoint(4,10) global-cursor= QPoint(-3192,326)
MainWindow::dumpGeom \\.\DISPLAY1 geom= QRect(-3196,294 256x214) dpr= 2