Qt Quick 2D Renderer: Partial scene update might erase items at fractional coordinates

Code

import QtQuick 2.15
import QtQuick.Window 2.15

Window {
    id: root
    width: 400
    height: 300
    visible: true

    Rectangle {
        id: box
        width: 100
        height: 100
        color: box.go ? "green" : "red"

        property bool go: false

        Rectangle {
            id: line
            y: 50.01 // Any non-integer value here will trigger the issue, say 50.01 or 50.99
            width: parent.width*2
            height: 1
            color: "blue"
        }
    }

    Timer {
        interval: 1000
        repeat: true
        running: true
        onTriggered: box.go = !box.go
    }
}

Steps to reproduce

1. Build and run the code above, with the environment variable QT_QUICK_BACKEND=software
2. Observe the GUI over a few seconds (as the Timer toggles a few times)
3. Resize the window
4. Observe the GUI over a few more seconds

Expected outcomes
While "box" toggles between red and green, "line" remains unchanged (blue) and visible at all times

Actual outcomes

• "line" is visible at startup
• When "box" changes colour, the portion of "line" that overlaps with "box" is erased and doesn't come back
• Resizing the window temporarily makes the whole "line" visible again, but the issue re-occurs when "box" changes colour

Notes

• Changing line.y to an integer value produces the expected outcome
• Using a non-software backend produces the expected outcome