In a Canvas item, calling getImageData on the context2D javascript objet, doesn't return data independent of the screen's DPI.
It works if the app doesn't call QGuiApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

It also works in Qt 5.7 and below.
In 5.8 you have to call ctx.getImageData((mouse.x*Screen.pixelDensity/2 ...etc.
Reproducible easily on Android and iOS, not on my Windows because it's not high DPI but maybe on a high DPI windows...