ToolButton stretch icon to background

In Qt 5.7 I should use contentItem instead of label. The following code not works:

ToolButton {
    contentItem: Image {
        anchors.centerIn: parent
        source: "icon.png"
    }
}

The image is stretched to button size, which could be bigger (for example, 48x48). In the gallery example, fillMode: Image.Pad added to Image. It seems to work, but only on my desktop.
On my phone with ~320 dpi screen and auto hidpi scaling enabled (resulting in 2x scaling) the image is either stretched, or stretched and has wrong position (when fillMode: Image.Pad is used).

The solution/workaround is to wrap it into Item and center on parent:

ToolButton {
    contentItem: Item {
        Image {
            anchors.centerIn: parent
            source: "icon.png"
        }
    }
}

The fillMode: Image.Pad should not be used here as the icon would be broken again.

Not sure, is it intended behaviour or not, but as gallery example is broken, I decided to create BR.

BTW, icon images in gallery is 20x20. I don't know, what size should be for other themes, but for Material it should be 24x24. Here file selectors based on theme would help