There is a mismatch between DDP and DIP coordinates when translating the event root location. The root location is in DDP and it's translated by the DIP origin of the window. The root location is to target events to the correct server window for a display.

https://cs.chromium.org/chromium/src/services/ui/ws/platform_display.cc?rcl=1477653611&l=198

So when device_scale_factor!=1, event targeting will be broken for secondary displays.