Software renderer: Visual glitches when moving things around a Flickable

Code

import QtQuick

Window {
    width: 400
    height: 300
    color: "black"
    visible: true

    ListView {
        anchors.fill: parent
        clip: true
        model: 60
        delegate: Item {
            width: ListView.view.width
            height: 65

            Rectangle {
                anchors.bottom: parent.bottom
                width: parent.width
                height: 1
                color: "red"
            }
        }
    }
}

Steps to reproduce

1. Add `QQuickWindow::setGraphicsApi(QSGRendererInterface::GraphicsApi::Software);` before constructing the QGuiApplication
2. Build and run the code above for WebAssembly
3. With the browser at 100% zoom, drag the ListView contents around
4. Zoom in to a non-round level (like 110% or 130%), then drag the ListView contents around

Outcomes

• At 100% zoom, we can see light grey glitches around the red lines as we drag the ListView contents
• At 110% zoom, we can see phantom artefacts/lines left on the screen (both in red and light grey)

Notes

• The hardware renderer does not have this issue
• This likely has the same root cause as QTBUG-133368
• The example above uses WebAssembly to make it easier to see the glitches. Other platforms also show the glitch, but in different colours:
  • The customer reported this on embedded Linux (linuxfb), where the glitches are black (so you must give the window a white background to see the glitches clearly)
  • On Windows 10, the glitches are the same colour as the line, which makes it look like the lines are changing thicknesses when they move around

• Zooming in 500%, or setting the environment variable QT_SCALE_FACTOR=10.0, also makes it easier to see the glitches