On some devices (like Nexus 5X, Huawei MediaPad M3) if one uses QML

ApplicationWindow {
    visibility: Window.FullScreen
    visible: true

    // color: "#FF000000" // possible "workaround"
}

with

QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

there will be some ~1px white border / stripes around screen.

Digging deeper I found it's Qt problem with rounding.

Huawei MediaPad M3 has 2600px x 1600px resolution with density (getDisplayMetrics().density) 3.0 what gives 853.(3)dp x 533.(3)dp. Qt rounds that down to 853dp x 533dp and multiplying it by devicePixelRatio (3.0) results in 2599px x 1599px - hence the white borders around the screen.

Nexus 5X is similar case - it has 1920px x 1080px resolution with density  2.625 what gives ~731.43dp x 411,43dp, Qt counts that as 731dp x 411dp what after multiplying by 2.625 results in ~1919px x ~1079px.

Possible "workaround" is to set ApplicationWindow's background color to black, so it's less noticeable, but it's still there (and what's more I'm not sure if this property sets OpenGL clear color or I'm doing unnecessary full screen overdraw here).

Another one I'm thinking about is to check on Java side those values, discover when such error will occur and then disable Qt auto scale and calculate own values, but it's troublesome and ugly.