"Dot artifact" in Qt Quick scene graph

[Edgar Chen]

After updating our application from using Qt 5.9.2 to 5.12.2, we noticed a very serious QML rendering bug. After displaying/editing text from multiple text fields, a permanent dot artifact will appear on the main application Window. The dot artifact won't go away even on screen redraw. See the following video for a test app that demonstrates the problem. Since the dot is small (look closely at the upper left of the test app), make sure the video window is at least 512x512 when viewing:
https://drive.google.com/file/d/1SWLtwaliHp78E420Js8CKz7FOj9_Ioca/view

The following is the QML code used to create the app shown above:

import QtQuick 2.12
import QtQuick.Controls 2.12

Rectangle {
    color: "grey"
    width: 600
    height: 600
    Dialog {
        id: "testDialog"
        width : 512
        height: 512
        contentItem: Rectangle {
            TextField {}
            TextField {
            y: 50        
            }
        }
        standardButtons: Dialog.Ok
    }
    Button {
       y: 520
       text: "Open Dialog"
       onClicked: { testDialog.open(); }
    }
}

The dot artifact seems to appear under the last position of the text field cursor. The color of the dot can be white or black or even other colors. We reproduced the issue using different video cards (nVidia, AMD) and even running under VMWare virtual machine. The bug does not occur if I switch back to 5.9.2. We are using Windows 10 version 1607, long term support branch. The bug also doesn't occur on Win 10 1803 or later, but we need to remain on 1607 due to other dependencies; and we needed some of the improvements in Qt 5.12.

I've submitted a bug against Qt 5.12. Has anyone also noticed this issue? Any idea what might be the cause? It's a serious issue for us since we are developing a medical image review workstation software and this artifact will render together with images we display.

Thank you in advance.

[jsulm]

@Edgar-Chen Just an idea: can you try to update the graphics driver?

[Edgar Chen]

@jsulm Yeah, we tried newer drivers, but no luck. Updating to a newer version of Win 10 did help, but as I mentioned, we need to stay with version 1607 LTSB for now due to other requirements. Also, we would like to learn the root cause since updating Windows might just hide the problem. Thanks.

[Izowiuz]

I have seen something very similar quite recently. I believe this is the same problem:

https://bugreports.qt.io/browse/QTBUG-74492

[Edgar Chen]

@Izowiuz Thanks! Yes that looks like the exact same problem. But the fix committed didn't include Windows 10. The fix comments mentioned the problem didn't occur under Windows 10, but we now reproduced here on Windows 10 1607 long term support branch.